Where did the mustang horses come from?

Answer 1

Mustangs are feral horses that escaped or were set free back when they were first reintroduced to the USA. The escaped horses mostly of Iberian (Lusitano, Sorria, and andalusian, Jennet) Bloodlines formed herds and continued to breed.

Answer 2

The Mustang horse is essentially a 'feral mutt'. They came from horses brought by the Spanish explorers that got lose, were stolen, or were left behind, this is why they are feral and not 'wild'. As for the breeds, there were many breeds that contributed to the Mustang, Lusitanos, Sorraia's, Andalusians, and other Iberian breeds were the first ones to become 'Mustangs'. After that everything from Shetland ponies to Shire draft horses have bred with the herds of feral horses.