The beautiful Morgan Horse (Equus cabullus morganensis) was adopted as the state horse of Massachusetts in 1970.

Morgans are descendants of "Figure," a sturdy little bay stallion owned by schoolteacher and singing master Justin Morgan of West Springfield, Massachusetts. Born in 1789, and later known by his master's name - Justin Morgan - this horse outran and outworked any horse that was brought against him. In 1821 the gallant little horse died at the age of 32.

Horses are official state symbols in 10 states:

Kentucky and Maryland - Thoroughbred horse
Vermont and Massachusetts - Morgan horse
Alabama - Racking horse
Idaho - Apaloosa horse
Tennessee - Tennessee walking horse photos
North Dakota - Nokota horse
Missouri - Fox trotting horse
New Jersey - "the horse"

Source:

Massachusetts State Symbols: Citizen Information Service

Links:

Best Horse Photos.com
State Animals - Mammals: 50 States List