The Zorse apparently exists, somewhere in the Galaxy. It is apparently a cross-breed of two Earthen animals, the Horse and the Zebra, which do not naturally meet. They were brought together by Humans and their offspring, after a bit of genetic tinkering, prospered into a fully-fledged species.
The Zorse was used as the mounts for colonisation missions to other worlds. The Zorse is now spread around the Human Empire, weirdly enough.