Hoppers are headless monsters resembling Isaac's body with a tiny hole where their heads should be. They appear in The Binding of Isaac and the Rebirth remake.
They can be found frequently in The Basement and less frequently in The Caves.
Behavior

Hopper from the original game.
Hoppers erratically jump around the room slowly, dealing Isaac with contact damage if he gets too close to them.
They can turn into a Flaming Hopper by touching a Fireplace.
Eternal Champion Behavior
Eternal Hoppers take longer to start jumping, but will jump directly towards Isaac with every jump and fire tears in the cardinal directions every time they land.
Notes
- In Rebirth, they can be spawned by the boss Mega Maw.
Trivia
Hopper on Isaac's Last Will.
- Hoppers and Leapers (also the Trites) are probably a reference to a Zelda enemy called Tektite[1], a spider-like enemy that also only moves around by jumping.