Since the Zonkey was brought up in another post here are a few cross breeds that I found on the internet. Which MANY of these breeds would never happen but thanks to mankind we have had some interesting combinations happen.
Wolphin (cross of false killer whale and dolphin)
Geep (cross of sheep and goat)
Golar (grizzly and polar bear)- This one does occur in the wild up in Canada where their territory crosses
Beefalo (cross of cow and buffalo)
Leopon (cross of Leopard and Lion)
Cama (cross of camel and llama)
Blood parrot Chiclid (cross of severum and red devil chiclid)
Zebroid (cross of zebra and horse)
Savannah Cat (cross of domestic cat and Serval)
Liger (cross of Lion and Tiger) which the opposite is Tigon. the male lion produces the Liger while the male Tiger produces a Tigron offspring
Zonkey (cross of zebra and donkey)
Zony (cross of zebra and pony)
And don't forget the Mule (cross of horse and donkey)
Here is one fact I found on one of the websites and direct quote from Hemmy.net
The top spot goes to ti-ligers/ti-tigon/li-tigons/li-ligers because it is a hybrid among the hybrids. It is a cross breed between a male tiger and a female liger/tigon or a male lion with a female tigon/liger. Do note that female ligers or tigons are fertile. They are extremely rare and are in mostly private ownership within a behavioural studies programme. In the case of ti-ligers, they have unusual striping where it breaks up and display a blotchy appearance. Since they are 3/4 tiger, their characteristics inhibit more of those of a tiger than a lion.
It seems like in most cases the cross between two different species ends up with an infertile baby where they are unable to reproduce due to the difference in chromosomes that each species have.