The term 'HORSE's Cousin' can be solved by two crossword solutions: ZEBRA (5 letters) and ASS (3 letters). Zebras are members of the Equidae family, which also includes horses, donkeys, and mules. They are native to Africa and are known for their distinctive black and white stripes. The ASS is a domesticated member of the Equidae family, and is closely related to the HORSE. It is used as a working animal in many parts of the world.