Yes, I think that they're uncommon because I don't see a lot of them. So what happens with them?

Once you get access to the Blue Building and have an open pet space you can adopt one there.

Woo! That is actually one of my goals! I plan on getting a Bumbus and then making it drink the Nuclear Potion I cooked for it! =^-^=

I don't yet have access to the Blue Building, but luckily I don't like Bumbus too much.