Einstein, The World's Smallest Miniature Horse

Organizations and registration requirements
There are two registries in the United States for Miniature Horses, the American Miniature Horse Association (AMHA) and the American Miniature Horse Registry (AMHR).
The AMHA was founded in 1978 and was dedicated to establishing the Miniature horse as a distinct breed of horse. The AMHR is a division of the American Shetland pony Club and was established as a separate registry in 1972. In the AMHA, Miniatures cannot exceed 34 inches at the withers (which the AMHA defines as located at the last hair of the mane). There are two divisions in AMHR - the "A" division for horses 34 inches (86 cm) and under, and the "B" division for horses 34 to 38 inches (86 to 97 cm).
Worldwide, there are dozens of miniature horse registries. Some organizations emphasize breeding of miniatures with horse characteristics, others encourage minis to retain pony characteristics.