Cannot tell you about horses, but I use a CBD with a low dose of THC in both a muscle cream and a vape pen. The muscle cream is MAGIC on my bad back and the vape definitely helps with relaxation and pain relief. I also use CBD infused treats for my arthritic dog. It seems to make a difference.

You can try CBD oil, but you should do lots of research as there are different strengths and it would be easy to give too much if you didn't read up on the exact dose and measure dose accurately, to a fraction of a ml. This resource can be helpful: https://www.vitalityhealthcbd.com/

THC toxicity is a very real risk.