When you hear the term "wild animal," the first thing that comes to mind are probably fierce, potentially violent animals like lions, bears or snakes.
But a bunny rabbit nibbling on some grass in the forest, or a crow flying around in a crowded city, are just as much "wild animals."
Once you remember that, it's only a small step to realizing that if animals could use a word to describe this state, that word wouldn't be "wild."
It would be "free."