Do not feed grass clippings to horses. Grass clippings begin to decompose almost instantly. Think of the heat of the lawnmower, plus the heat of laying out in the sun, etc. Not to mention the trip through the lawnmower, and all the other junk they are covered in.
I repeat. DO NOT. DO-NOT. Feed grass clippings to ANY animal. It's begging for a very high vet bill, and possibly a dead horse.
If you want to feed grass, either put the horse in a pasture or fresh pick it. Grass is good, but clippings can kill.
Wait! I'll fix it....