This boss just seems to chase after Isaac to get closer to him. Just avoid it and attack from a distance.
This monster looks just like Isaac, except it bleeds from it's eyes. When Isaac get's too close to them, they charge at him very fast, opening their eyes. Their eyes are actually just gaping bleeding eyesockets. When you kill them, sometimes they turn into a Gusher. They're kind of hard to avoid, especially with rocks and stuff blocking the way.
A Gusher is a headless monster that shoots blood out of it's headless neck. They wander around in confusion because they no longer have a head. They're usually pretty harmless.
The Mulligan is a monster that seems to be contaminated with a colony of flies. They run away from Isaac in fear and sometimes flies come out of their heads. When they die, they explode into a small colony of random flies. Usually harmless, unless there's a lot of them.
The Mulligoon is a lot like the Mulligan, it runs away from Isaac out of fear, but instead of flies, upon dying they explode like a bomb, and drop a troll bomb. They can also explode of their own accord. I've noticed that frequently when a bunch of Mulligoons spawn, there is a secret door they can blow open. I have yet to investigate this further.