A Shetland pony is a pony, not a horse. Officially, ponies are Equus caballus [domestic horse] under fourteen hands. Ponies have shorter legs and necks, heavier manes and tails, and heavier heads in proportion to their bodies, than horses. Their barrels are circular, whereas those of horses are oval.
Shetlands are one of the smallest pony breeds. They were originally even more stocky than they are now. The breed was selected for a more refined, horse-like shape when they shifted from being mostly draft animals to being mostly riding animals.