Fluffy

 are two Radroaches in the Abandoned Shack where you can get the Victory Rifle. They are caged, but you can open it to let them out. They will be hostile to the player. If you want to keep them alive in the cage don’t go in with a follower, as they will kill them. If you get too close to the cage, they can still attack you and deal damage.

Bugs
It has been reported that they can get out of the cage, even if the cage door is closed. (Confirmed PC, Xbox 360 and PS3)