There are a couple different tranquilizers used for horses (you are lucky one of my best friends has interned with a horse vet, so I happen to know a bit about this!).
*ACETYLPROMAZINE ( brand name ACEPROMAZINE) -
Dosage (for horses): .04-.08 MG/KG IM/IV
Duration: 4-6 hours
Time to take effect: 10-30 min when IMed, 2-4 min IVed
*DIAZEPAM (i.e. valium...in solution and meant for injection)
*KETAMINE (brand name KETASET, VETALAR) -- yep, horses get this stuff too -- and there are a number of painkiller/tranquirlizer and ketamine combo solutions.
*PHENCYCLIDINE (yep, PCP)
*PROMAZINE (SPARINE)
*TILETAMINE/ZOLAZEPAM (TELAZOL) -- dissociative and tranquilizer
Now, the only drugs of those I just listed that I know to have recreational value are: ketamine, tiletamine, diazepam and PCP. About the others I listed, I don't really know, though I'd guess that they aren't especially mind-blowing.
Other horse drugs that have recreational value: BUPRENORPHINE (BUPRENEX) -- a narcotic, BUTORPHANOL (STADOL OR TORBUTROL)-- another opiate-like narcotic, FENTANYL and a FENTANYL/DROPERIDOL combo -- a painkiller/opiate,
INACTIN -- a short lasting barbituate, MEPERIDINE (DEMEROL) -- a very recreational narcotic, METHOHEXITAL (BREVITAL/BREVANE) -- another short lasting barbituate, MORPHINE, OXYMORPHONE, NALBUPHINE (NUBAIN), PENTOBARBITAL Na (NEMBUTAL) -- a barbituate, THIAMYLAL (SURITAL) -- barbituate, and THIOPENTAL (PENTOTHAL) -- barbituate. Most if not all of these will be in injectable form.