the chickens, and other birds, are usually watching and waiting for the horses to stir up bugs and pull up a worm or two. Horses will graze through a patch of grass or weeds that have quite a few bugs in it and the chickens can catch what stirs up, then there are the bugs around the horses face they catch when opportunity presents itself. The disturbed ground may offer a worm or two. And once horses are accustom to chickens they offer some company and another set of eyes to watch for "monsters". Horses always seem to think there are "monsters" somewhere. And chances are, chickens know they are safer staying close to large veggie eating animals, because the chance of a cat or dog type predator coming around is somewhat reduced.
I do think they bond at times too. Many odd couples are formed by animals, did you ever see the clip about the elephant and the dog? That was great
I think chickens, or guineas, don't realize there is an
entire horse there, they may think the legs and head are all separate animals, in which case they have an entire flock to hang out with. You never know what all goes through a poultry brain, but I think the instinct to forage beside a large ground disturbing animal runs deep in their little brains and they know a meal will be turned up. Horses can stir up everything from spiders, ticks, fleas, flies, mosquitoes, moths, to small lizards and rodents..... all kinds of snacks.
Maybe the horses like the soft chirping and cooing the chickens do too.