Check with a good equine Vet, don't depend on this or any other board for a 100% correct answer... check with a Vet.
With that being said, I believe you can use any pyrethrin based flea and tick spray for dogs, not a permethrin based, it has to be pyrethrin based. Permethrin can be toxic, keep the pyrethrin based spray away from nose and eyes, if needing applied in that area spray it on your hand and wipe it on, don't spray it. I'm assuming the mare has ticks also... same thing, keep off the udder.
Good luck,
Alan