system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar: The Last Airbender] What's up with those animal names?

I mean, there are all these hybridized names, but we never see any normal names. I mean, we hear of koala sheep, but not koalas or sheep, for instance. No ostriches or horses, but they do have ostrich horses.
gpt
(I think the 4th wall reason is that the mixed animals are a joke about how, in Chinese, many animal names combine the characters for two other animals. For example, pandas are "cat bears". But, trying to answer in-universe:) 

This is a translation issue. 

In the Avatar universe, the language is much like Chinese, and many names for things are compounds of two simpler words. 

Now, no one is really calling a polar bear dog "polar bear" + "dog", since if either of those two animals exist, they are very rare and most people would never have heard of them. But, the polar bear dogs have two obvious traits: that they are large polar hunters,  and that they are loyal pets. There's a word to describe the large polar hunter  aspect (which is shared with the names of a few other animals that we'd say are half polar bear) and a word for the doglike aspect, and the name is naturally a compound of those two. 

Why are there so many animals with two aspects? This is a cosmological fact of the Avatar universe, that all creature have a dual nature. Most interestingly, of course, are humans, with a material aspect and a spiritual aspect.