Lawn of gloom

Common

 * Fume-Shroom
 * Gloom-Shroom
 * Puff-Shroom

Uncommon

 * Baby-Shroom*
 * Peashooter

Rare

 * Doom-Shroom
 * Hypno-Shroom
 * Cactus
 * Jalapeno

Once?
*: only becomes fumeshroom or gloomshroom
 * Imitater*

Every Flag

 * Flag Zombie
 * Target Zombie

Common

 * Zombie
 * Imp (alone)

Uncommon

 * backup dancer (alone)
 * Trashcan Zombie
 * Screendoor Zombie
 * Puff-Shroom Imp
 * zombie bobsled team (no bobsled and only 1 appears)

Rare

 * Disco Zombie
 * Balloon Zombie
 * Fume-Shroom Imp

Extremely Rare

 * Gloom-Shroom Gargantuar
 * Gargantuar
 * Zomboni

Summoned

 * trash imp (summoned by trashcan zombie)
 * backup dancer (summoned by disco zombie)
 * fumeshroom imp (summoned by gloom-shroom gargantuar)
 * imp (summoned by gargantuar and trashcan zombie)
 * zombie bobsled team (summoned by zomboni)