Creatures of Weirdmageddon | Gravity Falls Wiki | Fandom
Laurcanthrope — Why So Many Monsters/Creepy Stuff In Gravity...
Monster Falls Sketches 2 by purpleorchid-8863 on DeviantArt
10x4 collage of qwertyprophecy's "Monsters and Mysteries" series of posters for every episode (source and alternate layout in comments) : r/gravityfalls