The Grimler is a dragon mentioned in How to Break a Dragon's Heart.
Grimlers are a fairly typical Western style dragon. They have two wings and four limbs, a long snout and an arrow-shaped tail. They have long spikes running down their spines. They also have a horn and some whisker-like protrusions from their heads.
Grimlers have a rather strange biological ability—the females produce milk. Scientifically, this function makes them not a reptile, but a mammal (lactation is what makes mammals, mammals—mammary glands). Specifically, they would be grouped with echidnas and platypus as Monotremes, or egg-laying, milk producers. Had Grimlers existed, they would be quite difficult to classify taxonomically!
The Incomplete Book of Dragons tells us Grimlers have excellent hearing and scent abilities. They also have X-ray vision.
From the story in Book 8, Grimlers are also shown to form long-term mating pairs, have a strong maternal instinct, and are very involved in raising their young.
Grimlers are rated a "9" in disobedience, and therefore extremely difficult, if not impossible to train. They can however, as seen in Book 8, form relationships with humans under the right circumstances. Otherwise, they would gladly eat humans and joined Furious and the Dragon Rebellion.
In How to Break a Dragon's Heart, Alvin's mother states that when Grimbeard the Ghastly left Hiccup Horrendous Haddock II on the side of a mountain, a male Grimler Dragon found him, and took him to a female Grimler, which looked after him. They later adopted Furious, who became Hiccup Horrendous Haddock II's foster brother.
This reference book discusses Grimlers. These dragons are also mentioned in the Dragon Key list at the back of the book.