Horses seem comfortable around cyclists on upright bikes as they have a profile similar to rider on a small horse. By contrasts, the lower longer profile of many recumbents is similar to that of a cougar predator, so it tends to spook them.

The horse is comfortable around humans and if the humans a riding some other horselike critter, the horse may enjoy the chance to socialize. There is quite a bit of comfort in the peloton riders profile so in view of another poster's comments about horses running socially the opportunity to run with something comfortable my be the socializing running opportunity the horse enjoys.

I previously raised the issue of being spooked by the fear the peloton was fleeing a preditor. Many species will graze, feed, and water together keying on the other animals ability to spot differing preditor clues, so that is why I also asked about that concern.

Seems to me last year on OLN, I got a chuckle when I saw riders pass fenced in horses and saw a couple of the horses elected to run along opposite side fence while watching the cyclists-though they never lept their fence.