[mannykannot]

So much for my guess! BTW I am well aware that mustangs are not wild horses, which is precisely why I made the distinction in my original comment. When you say "any other unhandled horses", are you including truly wild horses?

[monocasa]

There arguably aren't truly wild horses anymore. The closest, Przewalski's horse, only exist in the wild as a population reintroduced from captivity. The captive population that they were reintroduced from very might well have been a more docile subset of the subspecies, making your question difficult to answer.