This is Catdog.
You know, the duo (or UNO?) that is half dog half cat, no butthole.
WHICH BEGS THIS QUESTION:
Here's one idea:
Here's another, more probable answer:
Most likely it is this:
Which would be why cat is seriously dreading that plate of mush dog is about to eat:
Or maybe, just maybe. There is no logic and the mystery will never be solved.
If we never find out how they poop, it doesn't matter because look how cute they were as babies.
Post your own theories about their defecation process in the comments!