"There are animals on the mountain."
To those asking why に is followed by は i will paraphrase an excellent answer i saw earlier:
Dont think of は as the subject marker. Rather, the topic marker. に is still correct usage for "on the mountain" (山に) but by putting the は after, it becomes the topic.
It would be a slightly different sentence if the は was used for どうぶつ because it would be something like this: "there are animals on the mountain" vs "the mountain has animals on it". An ever so slight difference.