Maiden
1) A horse or rider that has not won a race. 2) A female that has never been bred.  

Maiden Race
A race for non-winners.  

Mare
Female horse five-years-old or older.  

Minus Pool
A mutuel pool caused when a horse is so heavily played that, after deductions of state tax and commission, there is not enough money left to pay the legally prescribed minimum on each winning bet. The racing association usually makes up the difference.  

Money Rider
A rider who excels in rich races.  

Morning Glory
A horse that performs well in morning workouts but fails to reproduce that form in races.  

Morning Line
Probable odds on each horse in a race, as determined by a mathematical formula used by the track handicapper, who tries to gauge both the ability of the horse and the likely final odds as determined by the bettors.  

Mudder
Horse that races well on muddy tracks. 

Muddy (track)
A condition of a racetrack which is wet but has no standing water.  

Mutuel Pool
Short for "parimutuel pool." Sum of the wagers on a race or event, such as the win pool, daily double pool, exacta pool, etc.