Pink Elephants

The Pink Elephants are hallucinations that Dumbo has after drinking the Whiskey that the clowns drop in his water bucket.

Dumbo
During the film, the clowns are drinking whiskey after a long day of working. While most of them are drunk, they knock the whiskey bottle into Dumbo's water buckett. Dumbo and Timothy Mouse both drink the water and as a result, become drunk. Once the two animals have become loopy from the whiskey, they start to hallucinate that they are seeing Pink Elephants. These colorful hallucinations dance around and scare Dumbo and Timothy. Eventually, the effects of the whiskey wear off and the elephants fade away.

House of Mouse
In the episode "Mickey and Minnie's Vacation", while Donald and Daisy are in charge of the club, they book the Pink Elephants as one of their faulty acts (along with 101 Dragons). Fortunately, Timothy was able to get rid of the technicolor creatures.

Disney Parks
Pink Elephants appear in certain shows at the Disney Parks.

Animagique
Donald Duck encounters the elephants in the show briefly.

Fantasmic!
Pink Elephants appear as minor villains in the Disneyland version of Fantasmic!

Trivia

 * The Pink Elephants musical number is very controversial due to the inclusion of alcohol and drunkness.
 * The Nostalgia Critic ranked this moment as number 2 on his Top 11 Mindf*cks list.