Lavender and coconut oil is awesome to put in mane and tail bottle to help calm skin as well as hell hair grow thick and strong. I mix geranium for the moody horses. Also putting a lavender, chamomile, mint mixed spray on their legs helps with recovery after a hard ride!
Geranium is good for moody mares. Chamomile and lavender are calming. Ginger, Cardamom, and Anise oil kill bacteria.
Please be careful using essential oils with horses and never put directly on their skin. So many practice the 'raindrop' technique that can be toxic and create allergic reaction and hurt your horse.
Aromatherapists know, an effective way to absorb the benefits of essential oils is through the air passage ways. If used on skin, oils are always diluted significantly.
So - have your oils available for your horse to come up and smell. They'll tell you what they need.
We used aromotherapy oil with a mare that stressed out when travelling. It would calm down for about 30 seconds then starting kicking and carrying on again. Smells quite nice though. I once kept my horse at yard where there were crystals in all the water troughs for various things, not sure why, at to be honest scientifically it has no effect unless the 'crystal' leeches some mineral of something?
I know the placebo effect can be very useful on humans but I can't see it working on horses....
A friend does equine massage and as part of her work she uses essential oils. She did two types of massages on my mare over two weeks and left an essential oil for me to use called "Valor". I like it but I think there're different essential oils for what you want :)