Agreed - docks and nettles aren't attractive and the horses generally aren't interested in them. I've spent several days digging dock out of my pastures by hand because it was taking sections of them over. The goats don't care for it much either. I suspect that if I confined them to a small enough area they'd eat it but given their druthers, they'd choose other things.
The real problem (at least view from the angle of this question it is a problem) is that horses will eat a wide variety of plants. On our farm, only the goats will eat more things than the horse and donkey will.
Not sure exactly what you want to accomplish here - pretty much anything you plant that isn't unattractive to them (e.g., dock) or poisonous to them (e.g., Jimsonweed) they will likely eat. Early on in our life here on the farm I had some young trees in one of our pastures. They basically stripped those bare. My advice is to expect that they'll eat whatever you put in their fields.
You might have success putting up some electric fence (we use solar chargers on our here) to keep the horses from eating something you plant. I've fenced off a pine tree in one field using electric and it has effectively reduced their munching on the tree to only browsing on the tips that poke out above the fence line.