Work in 1988 (Forsten) suggests all modern domestic horses originate from Equus ferus a small statured European equid. Przewalski horses (E. Przewalski) have a different chromosome count (2n=66) compared to domestic horses (2n=64), meaning they are not the progeneter of modern domestic horses.
Enjoyed the entire post- right up my alley!
As for the equine- human connection, I believe it is a partnership, hopefully based on trust and respect. Horses do need to know they are safe with a superior leader and that goes a long way to their acceptance of a variety of situations. I also believe they have an ability or a 6th sense to understand things and intent in a way we do not, which gives them insight into people, their intent, and the environment.