The Fairly OddParents - Season 3 Episode 21 -

When Timmy wishes that his Imaginary Friend Gary was real, Imaginary Gary is instantly favored by Timmy's friends, family, and even Trixie. Later, however, it turns out that Gary wants revenge for Timmy abandoning him for real friends. Timmy cannot even unwish him because Gary is from his imagination, and thus is a part of him, allowing Gary to make wishes, too.