We've had Spirit, a 13.3 POA gelding for about 3 yrs. Now. He's in the middle of the pecking order in our herd, very calm, docile, doesn't kick or bite other horses when in hand or being ridden.
We brought in a new pony, 12 hd. POA gelding recently and put him in a corral next to the other geldings' pasture to let them meet and greet. No problem through the fence but when we brought them out together to ride this weekend , they immediately started trying to go after each other in hand and when being ridden. We were able to get them to behave for the most part but any time they were close they would start up again. We turned the new pony out in the big pasture and he was fine, rolling and nibbling on grass but Spirit pulled away and went after him. They went after each other like stallions, clashing, clamping onto each other's withers, kicking, rearing. It was amazing! I've never seen horses other than stallions in the wild go at it like that. I had to run at them, waving the riding pads I was carrying, to get them to break apart.
Do some horses/ponies just take an instant dislike to each other? Spirit doesn't act like that with any other horse. Any ideas?