According to Tuffnut, "the Cockatrice" is the unholy offspring of a Chicken and a Lycanwing. Secret is, he's been feeding it Glowing algae.
— Dragons: Rise of Berk
When Chicken goes missing, all the riders go and search for her. They all find Cockatrice and are scared of it, thinking it's Chicken. However, Chicken quickly shows up. Snotlout still has doubts about Cockatrice, but Tuffnut decides that "every friend of Chicken is my friend" and they all go back to Berk.
Cockatrice has the shape of an ordinary chicken. However, due to eating Glowing Algae, its feathers are black and purple and its eyes are bright red. along with a neon green beak.
Cockatrice is actually the name of a mythical dragon, half chicken, half reptile.