My horse is tense. I bought him a message curry comb mit at tractor supply for $5 and he loves it! Just find their favorite spot, and message away! He's definitely more relaxed now!
Smartpak sells smart calm altera pellets
I would recommend keeping them in a stress-free environment. Keep their feeding, watering, and turnnout times consistent as possible. Bond with your horse and get them to trust you completely. Let them know when they're doing the right think so they dont feel worried to always please you. If your horse feels stressed when riding, talk to them, and figure out whats bothering them.
If possible, keep them in the pasture and not a stall. Also ViaCalm works wonders with my horse! So does Mare Magic.