Grimora is a parasitic creature that appears about the size of a tiny mouse. It has a short tubular red body with a blunt tail. It has dragon-like wings, and a gray head section that appears to have gill slits. It has a round, sucking mouth filled with rows of teeth around the interior, similar to that of a Whispering Death. There appear to be no eyes. The Grimora are described as leeches, which are invertebrates. However, the appearance of their wings, suggests at least some sort of internal structural component. Their general appearance, mouth in particular, are reminiscent of Lamprey Fish.
Lampreys are jawless eel-like creatures with an internal skeleton of cartilage. That said, they do not have vertebrae, but instead have another structure just above the notochord called the "arcualia". They have fins, but lack pectoral fins. Lampreys are well-known for their parasitism of fish and have round mouths full of teeth. They attach to the flesh of their prey and bore into their body until they reach blood or other body fluids to feast on. To note: only some species of lamprey are parasitic, while others do not feed at all in their adult stage. Lamprey also spawn and spend their larval stage in freshwater. After metamorphosing into their adult form, they may migrate to the sea or into lakes. This parallels the Grimora in that they prefer fresh water and are found in a pool. There are some species of lamprey, however, that live in the ocean as adults. One other similarity between lampreys and Grimora is that some lampreys have toxic mucus and serum and must be thoroughly cleaned and cooked before eating. Lamprey is otherwise very edible, and is said to be very "meaty".
Salt appears to be the Grimora's weakness, though as seen in "A Grim Retreat" does not kill them outright. It is unknown why the creatures dislike salt, and subsequently salt water, but their aversion brings to mind slugs. When salt or a strong salt solution is applied to slugs, these invertebrates will have the water drawn out of their bodies by osmosis, dehydrating the creature.
Grimora presumably feed of a dragon's blood, though it is not specifically stated in the episode, "A Grim Retreat". It is also not mentioned if they prey on other animals, or just dragons. They seem to prefer to attach themselves around the head, neck and shoulders, of dragons. It is also possible Grimora release a numbing agent to prevent dragons from being aware they are being bit, like a tick.
Once attached, according to Fishlegs, the Grimora produce a toxin that increases wildness in dragons. This toxin takes a while - at least several hours to half a day or so - to start effecting a dragon. The very first symptom, however, is increased itching and scratching around the head and neck, presumably from the presence of feasting parasites.
Once the Grimora toxin effects the dragon's behavior and nervous system, the pupils constrict, sometimes unequally (Anisocoria). The toxin also causes hypersalivation. The saliva is thick and glowing and varies in color from dragon to dragon, appearently depending on the color of their breath (for example, Toothless' saliva becomes glowing purple, like that of his plasma blast). It is interesting to note that when the dragons ate Glowing Algae, their bodies glowed in similar colors (seen in "Fright of Passage").
Behaviorally, the dragon becomes aggressive and highly driven to hunt. Tame dragons may turn on their riders and hunt them doggedly. Infected dragons don't seem to attack each other, but appear willing to attack non-infected dragons. Dragons also appear to develop hydrophobia - fear of water - especially to salt water, even though the same dragon uninfected with Grimora may enjoy the water.
The disease created by the Grimora toxin in some ways mimics Rabies, though Rabies is caused by a virus, not a toxin. The virus causes inflammation in the brain and multiplies in the saliva glands, both of which can cause a variety of additional effects. Rabies disease can be described as "furious" or "dumb", with the "furious" form being more common. The infected animal displays erratic and at times aggressive behavior and tends to bite more, thereby transmitting the disease. The "furious" state also produces excessive salivation and sometimes hydrophobia. The "dumb" form is so-called because the infected animal is partially paralyzed and can't do much. It is possible that the Grimora toxin causes some inflammation in the brain.
Treatment for Grimora infection is simply driving away the parasite with salt or salt water. The toxin it produces does not appear to be very stable, as the effects diminish immediately after the parasite leaves. It is unknown what the fate is of a dragon that is unable to remove Grimora.