Jump to conclusions much...
We did do the introduction period. We are not new to introducing horses into the pasture. We had the new horse seperated from the others and he was doing perfectly fine. They were socializing over the fence and he didn't offer to kick or bite or anything. There was no squeeling or anything. We even talked to the previous owner of the place he was boarded and she said he never acted aggressive at all, he never tried to mount their mares, nothing. So because we had no problems after the introductions we let them all out together. They seemed to get along alright at first. My friend left for work and came home and he had mauled my horse during that time. We had no way of knowing how he was going to react.
I don't mean to jump on you but we didn't actually do it wrong. There was no indications that he was going to be aggressive in the beginning, and he doens't have any past of aggression at all. But don't worry they are seperated now, my horse and our other old gelding are in the side pasture and the mare is out with the new horse. They don't have ownership of him yet and therefore don't want to put him alone because he might hurt himself trying to get to the mare. Since he isn't hurting the mare we left them together, she going home next week anyway. Hopefully once the mare is gone the new horse will stop beating up the other two geldings.
Nothing serious happened to my horse, he is ok and is healing up fine. He is a little off in his movement but isn't in pain. He is trotting the fence line for hours so we know he isn't lame at all. It was an accident, it wasn't anyone's fault.