Monster Mania.

When his aunt mysteriously vanishes after spending the night at her spooky mansion, 10-year-old Brian McKenzie meets his first monster—clumsy, big-hearted Boo Marang—who jumps from a bedpost and whisks the young boy off to Monster Mania, a land of crazy monsters (located in dark closets everywhere). He helps the monsters battle the evil Osh and a variety of other monsters.