Cocaine. Although the term is often used to describe a multitude of other illegal substances, it was originally used as a slang term for the stimulant-drug cocaine (a processed extract of leaves from the Erythroxylum Coca plant).
"Lets ride the white pony" <let's use cocaine>
"Can you ask your buddy if he's good for the white pony?" <would you ask your friend if he has any cocaine to sell?>