Anything that's not human can be called "it". Anything that's biologically male can be called "he", and anything that's biologically female can be called "she". Animals could fall into either category. They're not human, so they can be called "it". They can also have a biological gender, so it's not incorrect to call them "he" or "she" (depending on the animal of course). We usually only call animals "he" or "she" if they're own pets. This makes sense: if you see an animal that's not yours, how would you know if it's male or female? Even if we do know an animal's gender, we still call it "it" unless it's our pet. For example, it's obvious that a maned lion is male, but we would still only call that lion "it". You would only ever hear a wild animal being called "he" or "she" in documentaries or in poems. My advice is this: call any pets of yours or your family's "he" or "she", but call any other animal "it".

'It' can be used for animals, objects and almost anything. Referring to a person as 'it' is however insulting so try not to do that. :)