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