Hmmm ... I was wondering what the origins of the word "mustang" were, and why Arakawa might have picked it for Roy's surname (besides the obvious, it sounds cool). So I looked it up on Wiki and read something interesting (and probably purely coincidental) considering Mustang's ambitions: One use of "mustang" is to refer to a commissioned officer in the military who has worked his way up the ranks from being enlisted (versus coming straight out of military school).
I also did find out the answer to my original question; it's Spanish and comes from a Latin word meaning "an ownerless beast". ;)