DianaM, size does not matter with the horses with minis thing as much as disposition. I've got a 14.1 mare that beats every equine up regardless of size. Come to think of it, I had a 13.2 mare that would run backwards kicking with both feet at other horses once. I wish I had a video of it, since it doesn't even sound possible. A smaller animal would likely do less damage if there were an accident though.
