Mammoth Jack: Breed of donkey known for it's large size and height.
Manege: An enclosure used for training and schooling horses. Also called a school.
Mangalarga Marchador Horse: Breed of horse adopted as the national horse of Brazil.
Mare: Female horse aged four and over.
Martingale: Item of tack which consists of a neck strap which buckles around the horse's neck and another one which attaches to the girth at one end, passes through the neck strap and attaches to either the noseband (standing martingale) or the reins (running martinglae) at the other. Used to prevent the horse from raising his head above the level of the rider's hand and evading the rein aids.
Mealy muzzle: Oatmeal colored muzzle, such as that seen in the Exmoor.
Meconium: Firm, dark brown or black fecal matter passed by the foal shortly after birth.
Melanoma: Growth or tumor often seen in grey or white horses. May or may not be malignant.
Middleweight: A horse that is judged capable, by virtue of its bone and substance, capable of carrying weights up to 196 lbs.
Missouri Foxtrotter: Breed of gaited horse developed in the Ozark Mountain region of Missouri.
Mitbah: Term used to describe the angle of at which the neck of the Arabian horse joins the head and which gives the characteristic arched set to the neck.
Monday Morning Disease: Common name for Azoturia, or tying up.
Morab Horse: Breed of horse derived from crossing Morgan horses with Arabians.
Morgan Horse: Gentle and elegant breed of horse developed in the 1780's. The founding stallion was a bay colt named Figure, owned by Justin Morgan, from whom the breed gets its name.
Mountain and Moorland: Name given collectively to the native breeds of Britain. See also native ponies.
Mucking or Mucking Out: Daily stable chore which involves the removal of wet and soiled bedding and general tidying of the stable.
Mule: Offspring of a male donkey and a female horse. See also Hinney.
Mustang: Wild horse of the American West.
Mutton Withers: Withers that are wide and flat seen in horses such as the Quarter Horse, as opposed to the prominent, bony withers often seen in the Thoroughbred.