So far I've never thrown any ponies away. I've had them arrive covered in cat pee (twice), and smelling of cigarettes as well as mold. In fact I'm still, months later, working on a lot of ponies who arrived absolutely reeking of mold and filled inside with that mold: they could clear a room.
I have ponies with ears and parts of their hooves and horns chewed off, but I will try to save them all.