why are some horses perpetually fussy?

I stole a beautiful brindle thoroughbred off a lady who raced me (and lost, so I thought it was a fair deal). I was riding the exact same horse and figured it'd be nice to have its twin with me. The first brindle is zen incarnate, a gentle, fearless horse from the start. The second brindle is a demon from hell, always shouting and rearing and generally being a loudmouth nuisance. I have a level three bond with her so far - I expected she'd calm down at some point.

Are some horses just perpetually fussy? Is it their personality? Or is this some sort of cosmic revenge for stealing the horse in the first place?