A cross between a horse and a zebra is rare but by no means unheard of. Colchester Zoo in Essex has had three zeedonks - crosses between a Chapman's Zebra and a black ass - since 1983.
(Today's word of the day is zeedonk. Say it after me....)
A horse has 64 chromosomes; the zebra has 44. The zorse that results from cross breeding will have a number of chromosomes that is somewhere in between.