Mole

The Mole is a monster added in The Binding of Isaac: Repentance.

It chases Isaac by burrowing in the ground (characterized by small, moving rubble on the surface). When it gets near Isaac, it pokes out of the ground and fires 3 blood shots at him. It will then jump to a nearby tile leaving a Pit or Poop where it jumped.

Tainted Mole
The Tainted Mole is a stronger, distorted version of the regular mole.

It burrows through the ground, creating rockwaves in the direction it moves in. When it gets near Isaac, it pokes out of the ground, creating a ring of rock waves (similar to the Roundy) and fires 3 blood shots at him 4 times. It will then jump to a nearby tile leaving a Pit or Poop where it jumped.

Trivia

 * Moles will have their eyes open if encountered in a dark room, however this doesn't affect their behavior.
 * Their white eyes also can be seen through the darkness.