Hamsters, gerbils, mice, rats & guinea pigs all at the moment belong to the Rodentia order. Hamsters, gerbils, mice and rats belong to the Muridae family. (Some sources place hamsters and gerbils in a seperate family.) Guinea pigs belong to the Cavidaeii family.
Rabbits belong to the Lagomorph order, not rodentia.
Horses and donkeys both belong to the family Equidae and genus Equus.
Dolphins, whales and porpoises all belong to the order Cetacea. But not even all dolphins belong to the same family.
A lion and a housecat belong to the genus Felis. But a tiger belongs to the genus panthernae.
Conclusion: Our classification system is flawed and I just wasted a good hour of my time. If there did exist a hybrid of rabbit and cavy, I'd bet it'd look more like this:
I not only learned lots from this thread, but the interspecies page I clicked on gave me some really, umm, educational information as well :s Like "it's mean to hide a kitten in a puppies butt".. :s now THAT is something I didn't expect to read about tonight. LOL