The Yeti as it appears in the film
The Yeti (AKA Wardy Bear) is a giant monster that appears in the the film. It is summoned by Sex Bob-omb via a distortion pedal, during the music battle against the Katayanagi Twins. The Yeti is quite powerful and easily destroyed the Twins' snow dragons.
The Game... I lost again
The Yeti as it appears in the game
The Yeti also appears in Scott Pilgrim vs. the World: The Game. If all players use their taunts in multiplayer, they'll start a band attack and summon the Yeti, dealing massive damage to all enemies and costing all players some guts points.