maiden
Breeding: A female that has never been bred.
Harness racing: A stallion, mare or gelding that has never won a heat or race at the gait at which it is entered to start and for which a purse is offered.
Thoroughbred racing: A horse or rider that has not won a race.
middle distance
Broadly, from one mile to 1-1/8 miles.
money rider
A rider who excels in rich races.
morning glory
Horse that performs well in morning workouts but fails to reproduce that form in races.
mudder
Horse that races well on muddy tracks. Also known as a “mudlark.”