Jack: Male donkey.
Jennet/Jenny: Female donkey.
Jog: Western riding term for trot. Also used to describe a slow, somewhat shortened pace in English riding.
Joint Ill: Disease in foals caused by bacteria that enter the body through the navel, resulting in infection which spreads to the joints and causes inflammation, pain and heat.
Jumper: Type of horse suited to jumping and which competes in jumping classes.