Little Mikey

Mike Wazowski's teddy bear (known as Little Mikey) is one of the toys from Monsters, Inc. and Monsters University.

Monsters, Inc.
In his first appearance, Mike was pushing a lot of things including his teddy bear. Later in the middle of the film, Boo found him and tries to play with him, but he took it away angrilly. When Boo cries, Sulley sings the Happy Bear song, but she touches his hand, which causes the teddy bear to fly away from his hands.

Monsters University
In the film's first teaser trailer, Mike's teddy bear can be seen next to him.

Trivia

 * A possible version of Mike's teddy bear was seen in the Butterfly Room in Toy Story 3.
 * A teddy bear bearing a resemblance to Mike's teddy bear is seen as one of Violet's toys in The Incredibles.
 * Unlike many of the other toys in the film, Mike's teddy bear isn't monster-real.