Tom (tired of the computer)

sounds fake but okay

did we not warn you??

That works twice! Deer is an animal, but also the animal. Here's what I mean.
Old English had a word "dēor", ancestor of "deer". It descended from a Proto-Indo-European word meaning "living thing", and meant "animal". But at some point the English speakers started saying the "animal" word while meaning a deer, the "default animal" so to say. And that's how we have the modern meaning.