Horrorcow is captured by Hiccup who gives her to Fishlegs because he cannot catch a dragon himself. The horror bit is to make her sound fearsome, while the cow bit refers to the fact that she is not remotely dangerous and spends much of her time in deep sleep. Fishlegs suspects she is a vegetarian. She is very fond of carrots. She is a normal sized dragon, bigger than Toothless—who she is best dragon friends with.
Horrorcow is very gentle-spoken, as in Day of the Dreader and the fishing competition.
Horrorcow spent two weeks latched to Fishlegs' head after the dragon rebellion attacked. 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.
It is revealed that Horrorcow has been hiding in underground on Berk. After the war ended, she returned to Fishlegs.