PCP, or phenycyclohexyl piperidine, is not currently used as a horse tranquilizer. Rather, "horse tranquilizer" is one of the many street names for PCP. The drug was used in the 1950s as a form of veterinary medicine.
PCP was discovered in 1926 and used as an anesthetic for surgery. However, surgeons quit administering PCP to patients in the 1950s because the drug's side effects included postoperative psychoses and dysphoria. Parke-Davis created Ketamine in 1962 to replace PCP as an anesthetic for both people and animals. PCP is now considered a street drug that can be found in powder, liquid, tablet, leaf mixture and rock crystal form. Other than horse tranquilizer, some common street names for PCP include angel dust, ozone and rocket fuel.