The Ghostly Trio give Casper the Friendly Ghost a pill to make him as mean as they are.
Practical Effects
Hand-drawn evil transformation hits different than CGI ever could.
Sound
Jack Mercer voicing two Casper personalities—chef's kiss.

Director
Seymour Kneitel
Trivia, insights & behind the scenes
Jack Mercer, voice of Ghost #3 and Evil Casper, was also Popeye for 40 years—dude had RANGE.
This short aired during peak 'rebellious youth' panic era; even Casper wasn't safe from conformity anxiety.