The following monsters are hidden in a block in a city. Use the indicated monster to destroy the corresponding block in the city until the bonus monster is unlocked.

Leon: Harry's, Las Vegas. Use Boris.
Kingston: Harry's, Las Vegas. Use Kyle.
Jack: Harry's, Las Vegas. Use Sarah.
Marco: Pacific Heights, San Francisco. Use Lizzie.
Shelby: Pacific Heights, San Francisco. Use Cal.
Natalie: Chocolate Factory, San Francisco. Use Bart.
Kyle: Mayfair, London. Use Boris.
Icky: Mayfair, London. Use Wally.
Edwin: Mayfair, London. Use Wally.
Plucky: Turtle Creek, Dallas. Use Ralph.
Fifi: Turtle Creek, Dallas. Use Rojo.
Cyril: Turtle Creek, Dallas. Use Sarah.
Cal: Studio B, Los Angeles. Use Plucky.
Bart: Studio B, Los Angeles. use Shelby.
Amanda: Studio B, Los Angeles. Use Jack.
Rojo: Michigan Avenue, Chicago. Use Rhette
Harry: Michigan Avenue, Chicago. Use Cyril.
Nick: Michigan Avenue, Chicago. Use Squirmy
Croc: Technology Park, Hong Kong. Use Kyle.
Venus: Technology Park, Hong Kong. Use Jack.
Jill: Technology Park, Hong Kong. Use Sarah.
Wally: Garment District, New York. Use Ruby.
Rocky: Garment District, New York. Use Cyril.
Joe: Garment District, New York. Use Icky.

The following monsters are unlocked by getting all upgrades for the indicated number of monsters.

Squirmy: Get 1 monster all their upgrades.
Fabio: Get 3 monsters all their upgrades.
Sarah: Get 6 monsters all their upgrades.
Brian: Get 12 monsters all their upgrades.
Vern: Get 20 monsters all their upgrades.