The Powerpuff Girls (1998)

S2E17 Imaginary Fiend

At Pokey Oaks, a newcomer named Mike Believe struggles to fit in and quickly becomes the target of his peers' disdain. In an effort to cope with his loneliness, he conjures up an imaginary friend. However, this creation turns out to be more of a troublemaker than he expected, leading to unforeseen chaos.