Actually, marijuana has been used in horse medicine for a long time - it used to be used all the time to treat colic. And there's a growing body of evidence that marijuana can be a good alternative to bute (which eventually wrecks the liver) for chronic pain in horses. It can also be used to stimulate appetite and, in a slightly higher dose, induce sleep (some horses do get insomnia).
There's now a campaign to restore the use of medical marijuana in horses. I'd definitely like to see a study on how it works as a colic treatment or, again, as an alternative to bute.