Need advice for an aggressive older gelding.?

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I have two geldings, one is 24 and the other is 11. Recently, I had to move them to a new location as the owner of the pasture wanted to plant Christmas trees. They are now with a woman who has horses herself and is really good with animals. She had my two geldings in a large pen for a couple weeks while the horses got to know each other over the fence. Then she turned them out with the other horses. The problem is, my 24 year old gelding, who I have owned for 23 years, started mounting her mares, being really aggressive toward her geldings, and also toward my other gelding. My two have been together for 10 years. The aggression just wasn't fading so after a week or two, she put him back in the large pen. She decided to get rid of two of her mares, nothing to do with my horse, one of which was the boss mare. With her gone, she tried once again to integrate him into the heard. Now the aggression seemed even worse.

He shows no aggression to humans. I was up there yesterday and even had an inexperienced rider ride him around the pen after I had trimmed his feet. He was the same good old horse that I have always known. He has been around other horses several times in his life and has never shown this type of aggression. He is not proud cut if that is what you are thinking.

I'm hoping that someone here can shed some light on the situation. Feel free to send an email message if you would like more info. Thanks so much.
He is aggressive to her mares as well as all of the geldings.