This one probably doesn't need an intoduction. Great for cuts to keep the flys away. Also good for around the eyes and ears if your horse doesn't care for a fly mask.
FARNAM Swat Original Fly Repellent Ointment Horse Supplement, 7-oz tub - Chewy.com
All of the horse get this!
Probios Dispersible Horse and Livestock Probiotic Powder, 240g
My show horses get this!
Purina Outlast Gastric Support Horse Supplement, 40 lb. Bag