Translation:There are animals on the mountain.
This is why 'subject' is a bad word for talking about grammar! It's unclear whether people are talking about what the sentence is about, or what the agent of the verb is.
Think of は as the topic particle - it might also be the subject of the verb in the sentence, but here it's not. So it marks 山に, or on the mountain as the topic. "On the mountain, there are animals" versus "animals are on the mountain". It's a subtle difference but it changes the tone and emphasis
Things become more clear if the sentence is broken down:
山 + に (mountain + marker indicating in or on) = on the mountain + は (topic marker) = regarding "on the mountain"
どうぶつ (animals) + が (subject indicator) + あります (there exist [the subject]) = there are animals
Put them together and you get something translating roughly into "regarding on the mountain, there are animals" or more appropriately "There are animals on the mountain."
@leeviticus well that's the general idea anyway, there's probably a lot more nuance than that! I just wanted to give a general idea so people aren't as confused when it pops up in strange places
It's not that unusual though - in English we usually put the subject (as in, the focus) first in the sentence, or use vocal stress or constructions like 'as far as X goes' for emphasis when we put the subject elsewhere. The whole particle thing is unusual though, yeah - it's interesting!
@Medusa747 I think it's a subtle thing, the は adds emphasis to the 電車の駅に part, so instead of a simple statement of 'there are people at the train station' it's more like 'there are people at the train station'. There's an implication that there's something significant about it - maybe everywhere else is deserted, that kind of thing.
We use a change in speaking tone to add that kind of emphasis in English, Japanese can use particles instead. Way smarter and more knowledgeable people than me are talking about it here!