Animals > Equines > Mearas > Horses > Ponies

    Probably from the obsolete French poulenet, which is a dimunitive of French poulain for colt, from Middle Latin pullanus, from Latin pullus for the young of an animal, a horse's foal.
    These are horses smaller than standard, but not miniature.  "Pony" usually refers to one of several breeds of horses which are small, stocky, gentle, and have high endurance.  Broncos, mustangs, Indian ponies, and similiar horses of the American West have been called ponies.

Reference: Webster's Seventh New Collegiate Dictionary 

Articles on individual ponies from Tolkien's works:
Bill the Pony
Merry's ponies
Fatty Lumpkin 
Strider, rider Frodo