We tried putting our bred mare in with him (he is pretty respectful to her), but he started to get bossy about the food with her. We then put him in with the geldings, and everything seemed to be going well until dinner time - he wouldn't let them within an acre of the food (that is also the side of the field nearest the pen with the mare who he is very possesive of).
He has done wonderfully so far being penned on his own - has always seemed well balanced and emotionally very stable. He has actually always been a real sweetie penned on his own. Since trying him in with other horses, he is a bit more of a brat. He actually bit me when I was working in his pen because I was by the mare. I was not paying attention to him, and all of a sudden I had horse teeth on my shoulder - I immediately made it very clear to him that he was not head stallion (I was), but he has never acted like that to any human before - I was pretty angry about this change in behavior.
After seeing this change in him, I would just prefer leaving him penned alone (he can see all of the other horses). Any opinions on this either way? Anyone heard of stallions developing social/mental problems from being penned alone?