Sometimes even we humans just don’t like another human and choose not to socialize or play games with that person. Horses tend to do the same. With luck the 4th horse will also be rejected by the 2 more aggressive ones, and will bond with Rowan. You did not cause this issue and probably can’t fix it. The best solution is to have a buddy for Rowan. Look on the bright side: if the 2 aggressive horses are not chasing Rowan away at treat time, then Rowan is not being physically attacked.