Frickin' Horse

On Friday, I was putting a little girl up on one of our (previously) best kid horses at work. His name is Max. He's a cremello quarter horse and the quietest gelding in the world. When he had to have surgery last summer, Old Boss had to keep telling the vet techs that he hadn't been sedated yet. After this particular surgery to relieve the tension in a hind leg tendons, Max was laid up for quite a while. He was lunged 5-6 days a week at walk then trot as per the vets instructions until he was finally allowed to be used for pony rides and then back out on the trails.
Since then, he has developed some very naughty habits. He no longer goes out with the herd because we are afraid he will reinjure himself so he stays in one of our larger paddocks. For a while he was by himself, but now he has a friend - another quarter horse who was laid up for a cough for a long time...
He decided in the time he was by himself that he was quite the horse and should be the big boss. So when he went out on the trails he found it very entertaining to trot up to the horse in front of him and biting him in the butt. This is very scary for some children (obviously) so we started making sure he was at least a little tired before putting kids on him. We would guide on him in the morning and then the child could ride him. So he was getting better. We even put some schooling rides on him to get him out of this habit.
So on Friday, I put this little girl on him. She was fearless and had lessons once a week. She could handle any mischief he tried. He tried to trot she would pull him back to a walk and chastise him thoroughly. Gotta love kids like that. I walked around the front of him like I usually do to change her off side stirrup. As I walked past his face, with no warning whatsoever - no pinned ears or grumpy face or anything - he bit me. He grabbed my arm up by the shoulder and bit me. Hard. and then let go looking like nothing unusual had happened.
Bastard.
I was stunned. I stood there for a second as I realized that he had, in fact, just bitten me and what I could do about it with a child on his back. I settled on a smack on the neck but it didn't faze him. But really there was nothing else I could do without risking the girl.
This horse is wonderfully calm. I wish I could know what was going through his brain that prompted him to just bite me like that. Whatever it was, it was not cool.
Luckily he did not break the skin... I was wearing several layers since it was pretty chilly out. But he did leave a pretty good bruise. Which given the way I sleep with my arms up against my chest, I tend to punch and wake myself up in the middle of the night when I roll over. Ouch.
Frickin' horse.