She should be fine for a little while at least. Horses can bond with other species (hence the horse rider bond!
). Soda will play tag with one of my dogs and when I ride him alone he is always more relaxed when the dogs come with.
One question to ask though is are your dogs horse smart? Some dogs are really dumb about horses, some will just ignore them, and some will chase them. It really depends on the personality of your horse and the dogs. Is your mare the type that would bond with a dog (curious, friendly, playful) or that would see them as "predators"?