Mulefootedness does come from a mutation, one that is hereditary, hence the ability to selectively breed these hogs into a "breed," though they have been documented worldwide and into antiquity through archaeological specimens and documented by Aristotle in Greece. The condition does occur in the European (aka "Russian") boar population, but this isn't a throwback.
The breeding came about, in part, because mulefooted hogs did not suffer foot/hoof rot that regular, cloven hooved hogs suffered after being in wet conditions for prolonged periods of time, such as muddy pens. Apparently, their resistance to this led to a number of other beliefs that mulefooted hogs were resistant to cholera and other diseases, which was purely hogwash. They were reported to mature faster, be less expensive to raise, be good milk producers and good sucklers. Not only that, but their meat was considered to be superior and by the 1930s, there were hundreds of registered breeders. The need for such artisanal pork declined with the rise of the beef industry and now the breed is nearly extinct, considered to be a heritage breed.
As for ferals being domestic hogs turned loose, that is certainly one way hogs can become feral, but you have to understand that since colonial times, farmers have allowed hogs to free-range. Jamestown had to build a palisade, not to keep out Native Americans, but to keep out the feral hogs.
https://books.google.com/books?id=aGFTAAAAYAAJ&pg=PA282&lpg=PA282&dq=jamestown+overrun+hogs&source=bl&ots=_yZtd55Dm_&sig=RAj6AzOn8NMLRWZheqYHrmL4FOM&hl=en&sa=X&ei=2Kc2UYiwLK762AWR54HwBw#v=onepage&q=jamestown%20overrun%20hogs&f=false
I know that free ranging was still ongoing in the 1940s when my father was a boy. He would turn out the pigs to the bottoms in the morning and whistle them up in the evenings. Sometimes he would get all the pigs back. Sometimes he would get a few extra. Sometimes he would be down a few. Some of the extra were the neighbors' hogs. Some never came back.
Of course, hogs may become feral by simply escaping, which is a common problem. We have all seen it. Livestock gets out from time to time.