Why breed such a large animal into a tiny thing when you have goats, rams, and mules that can pull carts around narrow streets?
Well, Ponys are small anyway.
And they're very very stocky for their small size, so they're very strong.
Being smaller they also require less food and space, which is useful on the Shetland Isles, since they're so remote and often boggy.
But ultimately, I can't say. Small horses on Shetland were in use for at least 5000 years, and the Shetland is assumed to be 3000-4000 years old.
Poll