My dog loves horse poo as well! You can tell when he has been eating it, since his own waste becomes quite fibrous.
Dogs eating horse manure is just part of the life of owning both horses and dogs. Mine, do not do it too often, because I tell them, "drop it", if I seeing them doing it, but sometime a Mom's just got to close your eyes, and wonder what kind of "kids" am I raising. :)
Barring the ivemectin problem my vet said that horse poop is one of the best snacks a dog could have! My dog loves it and never throws it up.