Okay i'm going to attempt to give my best advice here, but it was a little confusing to read your post so correct me if i'm wrong, but it looks like you have two issues here. One is you feel pressured by your friend to put your horse in with another horse, despite the fact he has retained an injury in the past from such a situation. And second, you are wondering if a horse reacting in "Fear" of a bite is strange or unnatural behavior?
First and formost, NEVER feel pressured by anyone to do something with your horse that you don't feel comfortable about. This goes beyond simple horse mechanics and into positive community relationships. If this is your friend, than she should respect your concerns and realize that having both horses in the same pen, might be detrimental to both animal's safety. Most boarding situations have either single turn out or group turn out, depending on what your bugget is and your comfort level with turning out your horse with other horses. Remember horses are social creatures, and every single one of them are different in personality, but incorporate many of the same "Body languages" seen in all horses. Meaning some horses when not previously developed into a "herd situation" can have several days to weeks of "Fighting, biting and kicking" between horses, because it is their nature to search out their place in the herd. Each horse in a group is either submissive to another horse, or dominate over the entire heard. And every new horse introduced to a herd, must "feel" their way around the herd to realize their standing within the cast system. Unfortunately horses lack vocal reasoning, so they "talk" with their mouths, and this is characterized by biting or nipping. Their lips are like their thumbs, and when telling another horse that "You are submissive to me" they "bite" them as a way of asserting physical dominance over one another. Depending on the horse, some horses are particularly aggressive when asserting dominance. Have you ever seen two stallions fighting before? However, it is more common in a gelding to gelding, or mare to mare, or mare to gelding situation for a horse to kickout, bite, rear, or in some cases, strike at another horse to demand submission from the newcomer.
The good news is that after several days, or in some cases, weeks, the newcomer will settle down into it's new "Herd lifestyle," and "get along to get along" with the other horses. (With the occasional "test" here and there, if a particular horse wants to challenge its place within the herd.)
In your situation most likely what happened is that your horse was not experiencing anything abnormal but simply "submitting" to your friends horse. Two horses can get along quite well when there's a fence between them, and that is because both horses realize the boundary to their own "territory." Have you ever seen a horse in one single paddock charge another horse in the paddock next to it for walking too close to the dividing fence? This is because the dominate horse was telling the oblivious horse that this is "My space" and you need to stay away from it! As soon as said horse removes itself from the fence, both horses go back to being happy grazers.
The same applys in a herd situation too. Most likely your friends horse was used to being the dominate horse in its paddock (especially if it never had any other paddock mates, and went unchallenged until now), so when your horse was introduced, the other horse responded naturally by asserting its dominance.
Horse language is purely physical, and can be quite violent at times. I'm sorry to hear your horse got injured because of this reality, but in order to protect your horse from future injury, I would decline your friends invitation, and pasture your horse in it's own paddock (maybe with an empty paddock between them). Then you'd have two happier, healthier horses.
Again if I'm wrong correct me, and I'll amend my advice.
Last edited by SunnyBlack; 05-05-2012 at 10:39 PM.