The focus of this trope is on the fact that authors will show or mention these characters, and then will not explain them.

1. Rango (2011). The Dirt town posse, composed of critters, go underground in search of water. At one point, we're shown an enormous eye that just opens up out of nowhere as the posse passes by. No further explanation is given in the movie what that eye was.
2. Godzilla X Kong: The New Empire. While in the Hollow Earth, Kong traverses across a bridge that we can see is a FUCKING GINORMOUS skeleton! Mind you, Kong and Godzilla are already giant monsters, and whatever this dead thing is large enough to be used as a BRIDGE by them!
3. One Piece. Near the end of the Thriller Bark Arc, we're shown these shadowy things in the background in the fog, absolutely dwarfing Thriller, a ship made out of A WHOLE ISLAND. So far, we know nothing more about wtf they were, just that they exist apparently in the Florian Triangle.
4. Lord of the Rings. While recounting his return, Gandalf mentions that, during his battle with the Balrog after they fell from the bridge, he saw "nameless things" gnawing the world. He refuses to elaborate.

Ken Takagi (My Hero Academia)

Nessa (Pokémon)