i think it's because "cock" doesn't have a gender in english as opposed to romanian.
If you do not know if a bird is male or female, you should say, "Is it a cock?" You should not say "Is he a cock?" because he must be a cock. He cannot be a hen.
For random animals, "it" is common, but if you are talking about a pet, using "he" and "she" is fine. Here, "he" should be accepted.
Rooster is not common in English (British). Cock is how a male chicken is referred to.
The question prior to this: "El este un elefant!" and the correct answer was "He is an elephant!". Now this one has only "cocos" in place of "elefant", and the answer is not "He is a rooster?". Why "Is it a rooster?" Could be a girl, no?
Generally, a male bird is a "cock", and a female bird is a "hen". Male chickens are "roosters" (US) or "cockerels" (UK) or "cocks" (general). You could refer to one as "he" or "it"; but "she" is definitely not suitable (he cannot be a girl!).
(This answer could be made more complicated with regional variations and the using of different words based on the birds age but it is not necessary to go there.)