At one time alcohol was used to help kill pain and even render someone unconcious for surgery. Alcohol was probably used to kill worms also. When I got my dogs as puppies from the pound I wormed them. One puppies worms were still alive when they came out. I poured alcohol on them and they died almost instantly. Since according to the article worms can sometimes cause colic alcohol was probably used to kill the pain and the worms.
Yes, it was used to kill pain in the past, when there were no anesthetic drugs available. However, it would take a very large amount of alcohol to put the horse in a drunken enough state to prevent the realization that pain was occuring.
People used alcohol for a great many things in the past based solely on assumption.
There is a saying "The body often heals IN SPITE OF what we do, rather than because of it".