Answer:
You may be looking for the Catoblepas which is thought to describe the Wildebeest or Gnu. (from "Giants, Monsters, & Dragons by Carol Rose)
It may also be the Japanese Ki'rin or Ki'lin or even Ch'i Lin such as announced the birth of Confucius. It is described as haing the body of a large deer, the legs and hooves of a horse, the tail of an ox and a single horn on its head. The Ki'rin has dragon scales on it as well.
The Calygreyhound may appear to be such a mix but is part antelope, part bull, with the forelegs of an eagle and represents fleetness.
There is a heraldic creature that is part horse and part deer (and is not the zorse nor a mule) and it is in "Giants, Monsters & Dragons" but I can't remember what it was called to look it up. But that one has no ox or dragon attributes.