Time is of the essence.
1. As soon as possible, secure an airway using tubing about 1/2" in diameter and about a foot long. Push it up to the bone in the cheek. A longer tube makes breathing harder, so don't make it too long.
2. Inject 20 mg Dexamethazone ( a steroid) for inflammation -Intramuscular
3. Inject 500mg Banamine for pain- intramuscular
4. Apply a turnicut for the snake bite on the proximal side of the bite. You can use vet wrap or elasticon for the wrap.
5. Most rattle snake bites in horses are dry bites, so they don't contain much toxin, but they are loaded with bacteria. Administer an antibiotic as soon as possible. About 15 cc's ceftiofur -in a vein or the muscle.
FYI. Ivermectin is poisonous for dogs and has been known to kill small dogs