Rose Hips
Fruit of the rose plant, which is in the same family as apples, rose hips have long been used for their high vitamin C content. Rose hips have a tart taste that horses enjoy.
A study in 2008 found that horses fed rosehips at a certain amount showed a reduction in inflammation in horses with osteoarthritis.
https://thehorse.com/152675/herb-shown-to-reduce-inflammation-in-equine-study-2/
Chamomile
Chamomile has long been revered as a calming plant, often found in teas for relaxation for humans. It is also used as a calming herb for horses, too. The smell of chamomile reminds me of sweet apples.
Please note, some organizations ban the use of calming agents, so please double check your organization rules.
https://thehorse.com/186763/taking-the-edge-off-calming-supplements-for-horses/
Peppermint Leaves
Peppermint is absolutely a favorite for most horses! Peppermint is also a beneficial herb known for soothing the digestive system.
https://madbarn.ca/supplements-for-digestive-health-in-horses/
Himalayan Pink Salt
Salt is an essential requirement for nerve and muscle function, and encourages horses to drink water. We use Himalayan Pink Salt in our porridges, but the right type of salt for your horse is whichever they prefer!
https://madbarn.com/how-to-feed-salt-to-your-horse/
https://thehorse.com/110471/is-there-a-right-kind-of-salt-for-horses/