There are going to be pasture bruises until the horses learn to get along. Horses are herd animals and there is a structure within the herd. The constant switching of herds does more harm than good. Unless a horse is out and out nasty, they tend to settle down within a week.
I truly wish people could watch the interaction in a herd sometimes. Especially those that think their horse is the poor picked on one.
I couldn't agree more! I tell people whose horses are new to our barn that their horse is likely to have marks on him the first week, as the horses have to figure out their pecking order. Most people who actually have had horses in their own care KNOW this... the people who've only ever boarded their horses seem to be that only ones who seem somewhat clueless to this (this is definitely not true across the board...).
We are very diligent about keeping herds that get along together, but the first few days usually involve some squealing and turning of the heels. One horse is ALWAYS going to come out on top, and the bigger the herd, the less likely the "alpha" horse is going to be yours :/ The kind of aggression that makes us put a horse by itself is when they back into another horse and don't stop trying to kick, or trying to bite/strike out without stopping.
If the horses are getting to know each other, chances are pretty good that some nipping will occur, and sometimes, one horse nips harder than the other :/ resulting in an ugly mark that will likely heal over (and isn't truly a scar, just a wound that is still healing) without issue.