I did not train my mustang myself. I have been retraining and refreshing her but that's all. I met the man that trained her and he is a very tough cowboy. He said she was fairly easy to train but that he had to use a firm hand on her. When I purchased her she had been sitting for a year and I haven't had to be firm with her. I would never in my life handle her the way that cowboy did.
I did train both of mine myself but it was a very different experience with both of them. Koda was as easy as any stock horse I have ever trained. Dobe, on the other hand, was extremely reactive and had a very high flight reflex. For the first year I rode him, I had to be very aware and careful at all times because if he saw something that scared him even a little, he would spin and bolt. Not to the point where he wouldn't stop, I could get him shut down immediately, but that was always his first instinct, to turn and run. I can imagine that most mustangs are just like most stock horses, there are some that take to training like it's the easiest thing in the world and there are some that are challenging as all get-out.
Yup, the ponies of Assateague Island came not only from Spanish galleons that sank hundreds of years ago, but they've also infused more recent domestic ponies into the herds to add more diversity to the bloodlines.
The only true wild equids to be found do not live on the North American continent. Anything here in the U.S. Is feral, not truly wild.