...but wasn't kimono the main form of wear back then? Not very practical on horseback, I can see how hakama could be introduced for horse riding.
Exactly! You could say that riding horses on the saddles we use sort of invented pants. Ask most any Scotsman (or some construction workers nowadays), and they say a kilt is much nicer to wear, leaving the legs free to move. Leggings were always something needed for hunting and riding, two things that may have contributed to the pants being worn by a warrior class.
A couple more points to consider: Japanese clothing follows a very rectangular pattern and seeks to form the body into a uniform pattern. This could account for the more voluminous nature of the hakama, which also has the ability to be both warm and cool when circumstances dictated. Hakama worn in the presence of high ranking officials were supposed to reduce movement (possibly a form of self defense?) while creating a greater sense of deference from those wearing them (they were hard to move around in, requiring focus and poise). Look at some of the examples that still abound in the kabuki....
The urban legends of "hiding one's feet" may persist despite more probable reasons, but one final rejoinder to them might be this: If someone belonged to a culture that dressed this way, don't you think they could tell where someone's feet were through familiarity with the medium (clothing)? As my teacher likes to point out, your feet are hooked to your ass, and people can still see THAT
plain and simple while wearing hakama.