It is the gnu (often known as a wildebeest or wildebai).
GNU
rat
bee
gnugnu -----> gun
U => Ewe .... a female sheep B => bee ... not a mammal but an animal nevertheess,
Fox-Ox
Ass
A herd animal that forms the plural by changing the 'y' to 'i' before adding the 'es' is pony - ponies.
An animal that has horns and a 3 letter name is an elk. Elks are a type of deer that can be found in North America, as well as Asia.
Caribou (Rangifer tarandus) is the North American name, this animal as known as reindeer the Arctic.
A name for a three-legged animal could be a "tripod."
He is refered to as a cutter or a good cow pony.