I have 3 horses now, 2 mares and one gelding.
One mare, Choo, is a 34" pony, 7 years or so old. When we added the new mare, Choo would threaten her through the fence. Baring her teeth, biting at the fence between them, presenting rear etc. They did become good friends however. Before we added the gelding to our little herd, Choo and Jaz would stand quietly together and groom each other. In spite of her small size, when we added Jaz, Choo became the lead, and the bigger horse has been happy to follow Choo. Choo loves to explore new places and exploring is more important then staying close to another horse.
The other mare is 13 HH and about the same age. Jaz has a quiet, calm personality and doesn't like to wander around alone. While the two mares were kept separated so that they could get to know each other through the fence, Jaz would always stand quietly as close to Choo as she could get, in spite of Choo's threats etc. Jaz would stay there all the time except when eating, drinking or when we were with her. She's never shown any aggressive behavior at all and would move off from Choo's threats.
The gelding is 22 years old, has skin cancer on his male bits and justs over 13 HH. He's very calm and gentle except at the food bowl. He'll dominate the food bowl but they have 4 feeding stations spaced about 50 feet or more apart. He's the newest and I don't know him very well yet, but he seems to be curious and friendly to other horses as long as they're not eating 'his' food.
We just started to let all three spend time together. The two mares have been spending time together for a couple of months now and its gone so smooth and easy. Choo leads, Jaz follows and at the food bowls, they both stay calm and friendly. Choo loves to flirt with Jaz when she's in heat and Jaz just loves having a horse friend.
We've had the gelding, Little Man, getting to know them through the fence and just recently let all three spend time together. Choo and Jaz are now fighting with each other. They squeal at each other and swing rears around to threaten each other. Both are presenting rear to Little in a mare-ish way...tail up etc. Jaz and Choo both clearly prefer Littles company to each other now.
Sorry for the long post, but wanted to give enough background info.
So at any rate, I'm worried about little Choo getting hurt as her and Jaz work things out. Both are treating Little like an intact stallion I think? I'm uncertain why the two mares are acting hostile to each other now. I've thought about separating all three, or maybe rotate which 2 are together for a bit so that all 3 can bond together. My goal is to eventually have all three together 100% safely. The turn out area I want them to share is about an acre, so theres enough space that they don't have to be close. Right now, when all three are out together, Jaz and Little keep close to each other and Choo stands off from them, but any time Jaz is a bit away from Little, Choo rushes in and starts kissing his legs, presenting rear, lifting tail etc and Little happily grooms her back. Then when Jaz comes closer, Choo squeals and threatens Jaz and Jaz threatens right back now. Until Little was added to the mix, Jaz would simply walk off a few steps and quietly look at Choo or ignore her.
Any ideas how to handle this so that the little horse stays safe and all three can eventually be turned out together? I tried returning to Choo and Jaz together with Little separated by a fence so that they could bond better before Little is added but even though that worked beautifully before, now Choo and Jaz are at odds even when Littles separated from them by a fence. I can separate all three or rotate which two are together, but I really want to be able to turn all three out together.