"Seeing pink elephants" is a euphemism for drunken hallucination.
Jack London, describing one sort of alcoholic in John Barleycorn, writes that he "is the man whom we all know, stupid, unimaginative, whose brain is bitten numbly by numb maggots; who walks generously with wide-spread, tentative legs, falls frequently in the gutter, and who sees, in the extremity of his ecstasy, blue mice and pink elephants. He is the type that gives rise to the jokes in the funny papers."
A reference to pink elephants occurs in the 1941 Disney classic film Dumbo in the song "Pink Elephants on Parade".
In one particular Simpsons episode, Barney Gumble, under the influence of LSD, has a hallucination about a monster. He quickly drinks a beer, which causes a pink elephant to appear and destroy the monster.
