My cousins brought their three Mustangs (well, two Mustangs and one Mustang/Quarterhorse cross) down over the course of two days... mainly because they couldn't catch the youngest one on the first day, lol.
They've been in the pasture with my horses for roughly three days now, and so far things seem to be getting along fine! They've formulated two seperate herds, and the pasture is large enough to allow it, but I'm sure in the next few weeks, they will slowly integrate into one herd.
My two horses (Gypsie and Dakota) keep to the 'Valley' (what I call the clear open area between two hills) and Gypsie is the herd leader in that little herd. She tries her best to keep Dakota away from the other group, with little luck, because I think he's bound and determined to 'meet and greet' the other group, lol. He's been a right pest to them, and he's already got two bite marks on his shoulder, but nothing bad.
My cousin's three horses (Wildman, Megan, and Drifter) keep to the 'Hills' (what I call the biggest wooded hill) and Wildman is the herd leader of that particular group. He's chestnut, whereas Megan is dark bay/faded black and Drifter is, I think, possibly grullo, I'm not really sure. He doesn't like my Dakota pestering them and following them around, but every time he pins his ears or even looks at Dakota wrong, Gypsie is right there between them, lol...
So yeah, things are going quite well.