Horse is also slang for heroin. A lot of theories have been generated as to why it's slang for heroin, such as the initial 'h' sound mimics the 'h' in heroin, or that heroin referred to as the horse because it has more kick than other drugs, but the reality is contained in the animal itself, and another vice of humans, gambling on horse races. This piece of slang is circa 1945, originating from horse trainers who used dosed their horses with heroin before races, because in equines, heroin is a powerful central nervous system stimulant.


The Slang of Sin, by Tom Dalzell, Merriam-Webster Inc.