Brain

Brains move around the room slowly, leaving a trail of Red Creep that does half a heart of damage to the player, though they do 1 heart worth of contact damage. They are relatively weak on their own, but can quickly corner Isaac when fought in groups. Two of them are spawned from MemBrains upon death. Brains can also be spawned by Oobs.

Pon
Pons are enemies which spawn when a Loose Knight is killed, and hop around leaving Red Creep wherever they land. Killing them will kill their corresponding Brainless Knight. They can sometimes spawn independently without a connected Loose Knight.

Poison Mind
Poison Minds behave like regular Brains, but they crawl diagonally and leave a trail of Green Creep.

Poison Minds float, passing over obstacles, and explode when killed.

Black Globin's Head
Black Globin's Heads behave like Brains, but they target Isaac directly, chasing him.

Trivia

 * The name of Pon is likely in reference to the Pons region of the brain, a part of the brainstem associated with respiration and other autonomic functions.