The Grimler is a dragon mentioned in How to Break a Dragon's Heart. They would gladly eat humans and joined Furious and the Dragon Rebellion. They can however, as seen in Book 8, form relationships with humans under the right circumstances.

Traits and Abilities

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! 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.

"The Complete Book of Dragons" tells us Grimlers have excellent hearing and scent abilities. They also have X-ray vision.

How to Train Your Dragon (series)

How to Break a Dragon's Heart

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.