Aye, and I agree, wish they would fit into the depot, I have 2 cheap houses that are almost complete full of nothing but PETS that roam.
From the way that I was told, from Denmum, it's a coding issue. Unlike a chair or table that doesn't move and just graphics. Pets have coding to allow them to move, which make them change in shape and stuff.
I have found that non-moveable pets (stationary) can fit in the depot. cuz I have some pandas/bears and some fish in the depot.