- "The "horror" bit was to make the poor creature at least sound a bit frightening. The "cow" bit was because for a dragon she really was remarkably like a cow. She was a large, peaceful, brown creature, with an easy-going nature. Fishlegs suspected she might even be a vegetarian."
- ―How to Train Your Dragon
Dragon Catching Day
As part of the Dragon Initiation Program of the Hairy Hooligan Tribe, thirteen boys enter the Dragon Nursery where thousands of young dragons are sleeping during Winter hibernation. One of these is Horrorcow, who is actually in the tunnels leading to the main cavern. As the boys had awakened most of the dragons, Hiccup quickly grabbed a dragon and stuffed it into his dragon-catching basket. This turned out to be Horrowcow. Realizing that Fishlegs did not have a dragon of his own and would get exiled, Hiccup gives the dragon to Fishlegs. She was still in hibernation. Hiccup braved the tunnel again and grabbed another dragon for himself, which turned out to be Toothless. Fishlegs names the Basic Brown female 'Horrorcow', to make her sound tough, yet 'cow' because she is nearly a vegetarian.
Training and Confronting the Green Death
Horrorcow was a relatively easy-going dragon, and listened - more or less - to Fishlegs using the 'yelling' technique of training. However, due to her unusual mostly vegetarian diet, Fishlegs had a hard time training her to fish and catch game.
Like all the other tame dragons on the Isle of Berk, she took the side of the presumptive winner in the fight between the Vikings and the Green Death, alternating between leaving and helping the novices defeat the giant dragon. She did help out with the feather-bombing plan, dropping gull feathers on the nose of the Green Death.
Red-Rage Dragon Attack
Horrorcow spent two weeks latched to Fishlegs' head after the red-rage dragons attacked at the Flashburn's School of Swordfighting. She finally let go after Fishlegs told her they were out of danger. After Hiccup led the second rebellion attack away, she was seen next to Fishlegs, who was looking down at Hiccup's helmet, fearing his friend was dead.
During the Dragon Rebellion
It is revealed that Horrorcow has been hiding in underground on Berk. After the war ended, she returned to Fishlegs.
Horrorcow is brown and of average size and appearance.
Horrorcow is rather gentle-natured for a dragon. She is not terribly brave and would rather not get into a lot of trouble, similar to Fishlegs. She does not have a high prey drive, as she is a vegetarian, and prefers carrots over fish.
- In How to Speak Dragonese Horrorcow is shown to speak Norse. However, this could just have been an error.
- Early concepts for the How to Train Your Dragon film included Horrorcow, but she did not make it into the finished product.