A cross between a zebra and a horse.
Here is a quote from http://www.geocities.com/zedonknzorse/basics.html:
“Zebras, donkeys, and horses are all members of the family equus – Equines. They are all slightly different in genetic makeup, but still all equines. Equines can be crossbred to produce hybrids. That is, horses have 64 chromosomes, donkeys have 62. The cross between a Donkey Jack (Stallion) and a mare Horse is called a Mule. The opposite cross is a Hinny. (Mules and hinnies can be male or female, either one, but are sterile since their chromosome count is 63).
So cross a ZEBRA to a HORSE and you get a ZORSE! Or a zebra stallion to a donkey jennet (mare) and you get a ZEBRASS (also called a zedonk or zonkey).“
Sources: 2007 Calendar ‘The 365 Amazing Trivia Facts’ and http://www.geocities.com/zedonknzorse/basics.html