Horse is
heroin. Those who ride the white horse are regular users, addicts, otherwise they'd be "chipping." Not to be
confused with "ride the pale horse" and "ride the white
pony."
At some point every
drug user, destitute and desperate, resorts to the cheapest accessible high. When you wonder what appeal
heroin has, it is that it fills this
niche. It winds up being the last alternative too, because when you ride the white horse, so often you subsequently die.