Ketamine is a drug that was developed for both humans and animals, and has multiple uses. It's most common applications are for use as a general anesthetic, tranquilizer, sedative, pain killer, antispasmodic, and anti-depressant.
Ketamine was developed during the Vietnam War as an alternative to a drug called phencycladine, which was known to cause hallucinations, neural spasms, and seizures. While not without its own side effects, it proved to be a safer method of putting soldiers to sleep when they needed to be in surgery. It also had excellent success as a local pain killer. It also became highly popular for the veterinary industry and is especially effective when used on large animals.
Sadly, it was discovered to have narcotic effects and had to be removed from public use in August 1999. Since then, it can only be obtained by prescription under a physician's care or by heath professionals.