Horses can be dangerous cos of their power and tendency to behave unpredictably...it's usually when you invade their space that they get pissed off. When trying to put a headcollar on one of my fillies she used to respond by trying to bite my face, that was til i twitched the little (she's 16.3hh and i'm only 5ft6") bugger, ha!

Of course, not all horses are dangerous!