Everyone knows that cats purr but there are other animals that do too. Purring isn't just about being content, it also serves to alleviate stress and pain. Being a cat owner I've seen a range of situations in which a cat will purr. Many years ago I found a badly injured cat and she was purring furiously which at the time confused me. I didn't realise that they purr even when they are frightened and hurt.
Several other animals: raccoons, squirrels, bear cubs, hyenas, bats, guinea pigs and rabbits even elephants purr too. But, they are not like a cat purr. Purring is an analogous sound produced by other mammal groups, but via a different mechanism, so some have argued it should be called something other than purring but it still serves the same purpose.