A 'pot-bellied pig' is a breed of pig that comes, I believe, from Vietnam. It is quite small, dark, and very ugly.
When animal or vegetable breeders combine two species, they are said to be 'cross-breeding'. This is abbreviated to both a verb (cross - to cross-breed) and a noun (a cross - the result of cross-breeding).
A pot-bellied pig crossed with a potato would small, ugly, ill-formed....