cats

Wheels that size shouldn't exist, and neither should those critter trail enclosures. I used to have pet mice and nothing about that setup would be appropriate for them, and I can't think of an animal that would do well in there. They're not even small enough to run comfortably in that wheel (they get similar back problems from running in wheels that small). But while hamsters are low on the "give a fuck" scale, mice are even lower.