My horses are turned out in large herds - 12-15 horses each. I frequently go get them in the pasture, or lead them in at bring-in time when all the horses are clustered near the gates. I expect all my horses to lead well - for me, this means that they follow behind me at an arm's-length on a loose lead - they stop when I stop, turn when I turn, and don't barge, or push or ignore where I am. If something seriously spooks them, they are never, ever, to run into me. This, to me, is a matter of my basic safety - horses are big animals and can do serious damage to a human - playing football with 1,200-pound linebackers isn't my idea of fun.
But there's a quid pro quo that goes with this. If I expect them to lead this nicely in circumstances where they might be threatened by another horse, it's my job to keep them safe. I've had a fair amount of experience working around groups of loose horses, and it's one of the most dangerous situations to be in, and a horse who is haltered and leading nicely is a sitting duck if one of the other horses decides to be aggressive. Leading a low-ranking horse through a group of higher-ranking horses is particularly problematic if you don't pay attention.
Before leading my horse up to and through a gate, I clear the gate by swinging my lead rope - I use a 10' cotton lead partly for this reason - as aggressively as I need to - body language also goes a long way. I make it very clear to every horse in the neighborhood that they are not, under any circumstance, to move in my direction - in fact they're to move away and not mob my horse as we go through the gate. This gives my horses confidence to lead along with me in the way I want.
It always surprises me how many people lead horses through problematic situations without paying adequate attention to their safety and the safety of their horse - I see it happen fairly often.
Today was a special case. It was extremely cold last night, with wind chills well below zero - in the 25-35 degreesF below zero range. So the pasture horses - the herd that lives outside 24/7 - were brought into the indoor arena with a heated water tank and hay bags. There are about 10 pasture horses. My horses are in a small, 11-stall barn (that is an old converted dairy bank barn). The access to the turnout pastures from this barn is through the indoor arena.
It wasn't too bad outside this morning and was supposed to get better, so I decided I would turn Red and Pie, who could care less about the cold, out at around 7:00 a.m. Since I was going to be leading them through a herd of strange horses - not ones that know them - I went a step further and carried a dressage whip to fend off any attention from the pasture herd. I swished the whip at them to make sure they understood I meant business, and they scattered and stayed clear as I first led Red, and then Pie, through the arena. Red at first was alarmed by the swishing dressage whip - he has a serious whip phobia, but I laughed at him and told him I wasn't after him, but the other horses, and he settled right down. (And even when he reacted to the sound of the whip, he didn't put any pressure on the lead line or move into my space.)
Missy and Dawn went out later, once the pasture herd was back out - I'd be hesitant to lead a mare through a herd of unfamiliar geldings, even with a dressage whip.
My message to my horses when there are loose horses around is that I've got their back.