I've worked mainly with Rocky Mountain horses, and I've noticed that they are very different than other horses.
Especially the stallion. I've been yelled at for keeping him with a mare when he's two. But I tried to explain to them that he's totally uninterested. 2 year old RMH stallions don't have those hormones yet. Or at least not the ones I know. Also the stallions in this breed seem to be easier to handle than the geldings.
I don't know if anyone else have noticed major differences in breeds.