From instructables.com, Papercraft Slime Monster Following Eye Illusion

From phillipfickling.com, Paper Toy Cthulhu

Papercraft Pudding Monsters 

Paper Toy - Cyclops Slime 

Paper Toy - Super Crate Box Skull Heads

Paper Toys Fin & Gill 

Papercrafts Paper Toys Monster Workshop 

From JustusProduct here another papercraft this time is PixelBlox Nightmare 

Frankenstein :

This Paper Toy was created by bob canada  and it's pretty amazing!

Billy Sweet Monster

Micro Monsters

Stalchild 

 Slurg Monster 

El Chupacabra

Monster Swan Ship

 Namahage Monster  

Karakasa le Monster

Albert Monster

 AWS' Piccolo & Critter

Official Monsters vs. Aliens