Vio a los elefantes

Hola Steph

We don't tend to use the "a" with animals if we're talking about them as in "generic animals", but we often use the "a" if there is an emotional or personal nuance, like in this case, where the narrator is expressing that affection towards the animals - something similar to when we talk about our own pets.

But you could have used it without the "a" and it'd be correct, as we don't specify any nuance here in the exercise.

I hope this clarified it.

Saludos

Inma