Dragons are unique and intelligent creatures, possessing many fantasy characteristics, but they are not completely beyond the scope of reality. Some characteristics include flight, flame production, intelligence, and more.
This section is the anatomy that almost all dragons share.
The scales of a dragon are quite different to any other scaled creature, not only do they serve as protection from nature's elements and physical attacks, but to look intimidating and/or to attract a mate. There seem to be two types of scales; regular scales (the small scales that line the dragon's entire body, apart from the eyes, mouths, and some other parts) and scutes (the larger, distinguishing scales on the body that is unique to only that dragon, like our fingerprints). An example of this are the large scales on Toothless' head. Dragon scales are extremely heat resistant, but not totally fireproof. Another example that Monstrous Nightmares scales are said to fall off like flaming missiles.
The dragon's wings main purpose are clearly for flight. However, they serve multiple other purposes such as fighting, protection, and forelimbs in order for some dragons to crawl (walk). A unique property of wings is their healing ability. Unlike our skin that repairs its self by growing over and scabbing, dragon wings are suspected of growing out from the leading edge of the wing and eventually dying and falling off when it reaches the edge. This is quite evident by the tattering at the edges of dragon wings where the damaged areas fall off.
Dragon eyes are quite similar to other reptilian eyes. Dragon eyes are naturally slightly slit, but when docile, the slit becomes rounder or oval. They tend to fluctuate depending on their emotions and the senses.
Dragons have a great sense of navigation; they are able to find their way to and from “Dragon island” and to the island where they lay their eggs. This amazing quality may be the same that salmon use to find the stream where they were born. Another thing used for navigation is the “purr” that dragons emit when in the presence of other dragons. This is evident in the movie when Toothless takes Hiccup and Astrid to dragon island. An incredible navigation tool that the Night Fury possesses is the use of echolocation as it was first shown in "We Are Family, Part 1 "
A dragon's skeleton is strong. It is able to withstand the greatest of impacts ranging from falling rocks to smashing into a sea-stack. They can take a beating. This is due to the fact that dragon bones are solid, unlike the hollow bones of birds, which makes birds very delicate. Dragon skeletons vary from species to species.
Though we have never seen the inside of a dragon's head, we can still make some accurate predictions on how it is wired. Dragons are highly adapted to their surroundings; they are an animal of senses: visual, audio, smell, touch and taste. They use these senses to survive in an ever-changing world. However, two senses stick out, visual and audio. These are crucial to their very survival, so they can eat, and avoid being eaten. Because of the heavy reliance of the senses, a lot of their brain is devoted to analyzing received data, and sending it throughout the brain and the other sensory cortices. Two of these sensory cortices are extremely intertwined, over stimulating one can affect the other: the audio cortex and the visual cortex. Too much sound can affect the vision of a dragon, compromising its aim. Also, this is shown through the echolocation of Night Furies, they are able to send out a pitch of X frequency and be able to construct a three dimension picture in its head outlining the surfaces.
This section is the anatomy of specific species and sub-species
The Night fury is a unique dragon, it is streamline and stealthy, built for speed and agility. It has a smaller body and large wings. This allows it to do a vertical takeoff. The great power to weight ratio allows for hairpin turns and fast ground speed. The body of a Night fury is jet black with lighter patches resembling a manta ray. The dragon is specially adapted for nighttime, large eyes, and echolocation used for navigation in the night and in dark places. The night fury’s pupils are able to expand to allow more light to be absorbed by the eyes; it is possible that the night fury has a tapetum lucidum< (reflective) layer at the back of their eyes to double the amount of light received. The Night Fury has eight ear-like appendages though only two act like ears, the other six are shown to vibrate while navigating to dragon island. The two larger ear-like appendages act like radar-like cat ears; they are able to pick off single sounds in the distance. It is also shown that the Night Fury has an exceptional sense of hearing, an example being that Toothless was able to hear Hiccup scream in the arena all the way from the cove. Night Fury flaps on their head can determine their age as shown by Valka. Their ability to breath fire seems a bit different from that of most dragons. Instead of accumulating some sort of gas or liquid in their mouths, igniting it and spitting, what they are doing looks more like taking a deep breath, "charging" it with distinctive sound, and firing something looking more like pure plasma. Its colour may be very bright, almost white, purple or blue. It may be fired in various forms, like some sort of short beam, exploding bolt or even a weak, slightly concussive blast (Like when he was sneezing, it did almost nothing aside from slight knock back). While in some cases Hiccup says that he has a limit of six shots, this is actually not sure, as we can see him shot many more of them from time to time. It is possible that this limit applies only to the battle typed fire blasts, and the weaker ones have different, if any, limit. When scratched behind the ears, their back spines split apart and this enables them to fly with greater agility than before. Sometimes, it can go into "Alpha Mode," where a Night Fury's back spines glow blue and a faint dark blue glow is emitted from its head and along its back.
The Night Fury is an extremely intelligent creature; it is able to understand visual signals and is able to pick up on verbal commands and phrases. The creature is able to pick out threats like Hiccup's knife. They are very curious creatures, they observe things they have never seen before, and they are shown to learn by mimicking behaviors like Hiccup's smile. Night Furies are territorial creatures, they are protective of their belongings, and their friends and they will not hesitate to put their lives on the line to protect them. They have emotions too showing sadness, happiness, anger, and a variety of other emotions, Night Furies are extremely loyal.
The Monstrous Nightmare is technically a wyvern (a two-legged dragon) though calling it a dragon is still correct. The dragon has two large legs that support most of its body mass It supports the rest of its mass on the hands on its wings (where all the wing fingers splay off) where it has a pad for walking and razor sharp talons for grip. The Monstrous Nightmare has an ever-burning pilot light within its self that is present from the time that it hatches, if this pilot light goes out, the dragon will get hypothermia. This is because the dragon is not able to create body heat without it. The hypodermic dragon loses the color in its scales as its blood rushes into its core to keep itself warm and to slow the loss of heat. The dragon will then become highly weak as not enough heat and oxygen-rich blood reaches its muscles. If untreated and the “pilot light” is not relighted, the dragon will die of exhaustion or hypothermia. The Monstrous Nightmare creates a gel-like fuel within its body (likely an alcohol because it is easy for cells to produce) which is then stored inside a large bladder. When the dragon needs to breathe fire the bladder contracts and forces the gel through its neck into the mouth. Somewhere along the line, the dragons pilot light ignites the fuel. The Monstrous Nightmare has a unique ability: it can set its self on fire. It is said that it coats its self in its saliva, yet the Monstrous Nightmare is never shown to groom, and if that is the case the dragon would not have any control over the flame on its body. So a theory is that the dragon has small “sweat” glands under its scales that secrets the gel on command. The dragon's pilot light then ignites the “sweat”. The dragon sports large horns on its head that varies with the dragon's age. The dragon's horns are unique to the individual dragon. It is possible that the dragons use its horns to compete for mates though is might be correct as we have seen Hookfang going against Titan Monstrous Nightmare with their horns.
The Monstrous Nightmare is a hot-headed, feisty dragon; these dragons are aggressive and if spooked will attack anyone in its way. Like Hookfang, they are maybe not as loyal as Toothless, but when their master is in trouble, they are not afraid to protect them.
The Skrill is a unique dragon. It is the first known dragon (that we know of) that breathes lightning instead of fire.The electricity runs all the way from the Skrill's wing tips to its tail and back up along its body to its mouth. The Skrill's body is also lined with large spikes along its spinal cord and head. It is unknown if the spines are fixed in place (by growing from the bone), or if they are attached to muscles and are able to lay down flat against its body. The spines on the skull are able to flex and move up and down, so they are likely attached muscularly. The Skrill's locomotion is similar to that of a Monstrous Nightmare in that it only has two legs and stabilizes its self-using the semi-rudimentary claw-like fingers on its wings to support its upper body. The color of a Skrill may vary depending on age, gender, and birth location, but from what we know, a Skrill is covered in light and dark gray scales, with purple patches on the back of its head and the top of its tail.
Skrills are hyper-persistent; long after a normal predator would consider a rival for its territory scared off, a Skrill will continue fighting it. This may be because of a small tactical part of its brain; its senses appear to be excellent, but as mentioned before, it will continue fighting a capable enemy even if it is outmatched.
The Hobblegrunt has a theropod-like appearance. It has a big, single horn that looks sort of like a Deadly Nadder's but is smaller than its head. It has very sharp teeth that are close together and point upward. It has a frill on the back of its head and eyes near the sides of its horn. Hobblegrunt's are found in many different colors, such as green, yellow, orange, and purple (similar to some other dragons, like Terrible Terrors). It also has small hooked wings, tiny arms, legs like a Nadder's, and a what seems like a long body. It is sometimes shown as slim and streamlined while other times it is shown to be fat.
Seems to be mostly friendly and affable.
The Screaming Death is a rare mutation originating from Whispering Deaths. It is an all white boulder class dragon with deep red eyes. It has a snakelike body like the Whispering Death and is also lined with spines. The tip of its tail is red and when it flies it seems to move its tail in a circular like motion. It has 3 rows of frills on the back of its head that shoot out fire when it spews a fireball rapidly like a machine gun which the frill shot may or may not be caused by some kind of chain reaction. Its movement like the Whispering Death is very similar to a snake. It has 3 rows of teeth that are aligned differently from a Whispering Death's and has a chin reminiscent to the Red Death. Its jaw doesn't close all the way and shows all the front teeth. some of which lock into each other. It can also tunnel underground either using its teeth to tear through the earth or it uses its thick skull which it can use to bash and destroy sea stags. When it first hatched it wasn't as big until it was seen the second time. It gets its name from its roar which can affect the flight of a dragon and cause it to lose focus like noise can confuse a dragon's aim.
The Screaming Death is ruthless. It attacks those who attack it (seen when Hiccup and Toothless shot a plasma blast at it and it chased them). It always comes back for revenge after it has failed. It is merciless and never backs down, although it was easily tricked by Hiccup and Toothless when it followed the light and crashed into the sea stack. However, it seems to get smarter every time it grows and did not fall for that trick a second time. In the end, the Screaming Death will really just do anything to get back to its birthing place.
Speed Stingers resemble a theropod dinosaur but with sail-like appendages on its head, back, and legs. Most are actually colored green, one has been seen with red fins and stripes. It has a barbed stinger on the tip of its tail with venom that can temporarily paralyze its victims. The paralysis may work similarly to the Flightmare's, but it injects the venom in the victim which enters the nervous system and cause every nerve to freeze, causing the muscles to freeze. It is shown that they also have orange eyes. They have 2 claws on their fingers and 3 claws on their toes (which probably help with traction on the ground). The tails may also help in balance since they run very fast and are great at counter balance for making quick turns. They have very sharp and pointed teeth that could help to tear into the flesh of its food. Their tongues are like other dragons but have two small bulbs at the tip. It is also revealed that Speed Stingers have later evolved by growing webbing on their feet for running across large bodies of water.
Their behavioral traits are unknown but since they "took over the village," they are most likely very territorial and will claim any land as their own.
The Red Death has an unusual skull that is very thick with a set of six eyes and long sharp teeth. Its tail is long and has thick with a mace-like club it that is used in combat. The wings are small proportionally sized, but the Red Death is shown to be fast enough to follow Toothless and Hiccup.
The Red Death is shown to act like a dragon dictator or queen, and is very destructive, crushing and eating dragons when it is displeased or when dragons do not bring it back enough food.