Is there a breed of horse from America with spotted rump?

Also the Pony of the Americas is a spotted & blanket coloured breed which was created by crossing appaloosa with shetland pony.

~Natural~

Yes, the Appaloosa.

The answer you are probably looking for is the Appaloosa - however you are confusing a horse breed with coat patterning, though the two run hand in hand as the colour of a horses coat is controlled by its genes. The gene responsible for spotting is the Leopard Complex (Lp) and different spotting patterns happen depending whether the gene carried by the parents is homozygous(dominant or recessive) or heterozygous due to the Lp gene being incomplete. A horse's coat pattern that has a spotted rump is known as a blanket spot but other spotting pattens include leopard spot, snowcap, few spot and snowflake. The Lp gene is also responsible for other characteristics such as stripey hooves, white sclera and skin mottling. Other breeds of horses also have the Lp gene - notably the Knabstrupper and the English spotted pony.