The sentence does not require a personal a in the sentence: Miramos los leones en el zoológico.
Explanation:
The rule is that we need to use the "a" preposition to separate the animal common name only in the case of a domestic animal common name like "el caballo" the horse "el perro" the dog. But in cases of wild animals like "león" we don't use it because the foundation of this preposition is proximity proximity to the human in bond not in distance. So we don't need an "a" preposition.
