Monsters refers to the many antagonistic creatures the player will encounter throughout the game.
Most monsters (with few exceptions) will damage Isaac by either touching him or shooting him with projectiles. Less frequently, monsters will spawn additional, lesser monsters, spread creep, or attempt to affect the player's movement. Along with the more common variety of monsters, champion versions can be found lurking about. These monsters are often like their common counterparts, but have a color tint or size change and drop basic pickups on death. There are also champion versions of most bosses.
Walks around the room, avoiding Isaac. Upon death, spawns several flies, which can be Black Flies, Attack Flies, or Pooters. Can suicide to explode and launch projectiles in the four ordinal directions while still releasing flies.
Walks around the room, avoiding Isaac. Upon death, spawns a Troll Bomb and fires four shots in the ordinal directions. Can suicide to explode, spawn a Troll Bomb, and launch projectiles in eight directions.
Slowly walks towards Isaac. Occasionally leaps off-screen and attempts to land on Isaac, leaving a puddle of creep and shooting blood shots in 6 directions where it lands. Can turn into a Blubber upon death.
Chases Isaac, dealing a full heart of contact damage. Upon taking enough damage, it collapses into a pile of goo, which can be killed. Will regenerate after a short period of time if not killed quickly.
Quickly chases Isaac, dealing a full heart of contact damage. Upon taking enough damage, it collapses into a pile of goo, which can be killed. Will regenerate after a short period of time if not killed quickly.
Chases Isaac, dealing a full heart of contact damage. Upon taking enough damage, it collapses into a pile of goo, spawns a spider, and leaves a puddle of slowing black creep. Will regenerate after a short period of time if not killed quickly.
Chases Isaac, dealing a full heart of contact damage and spawning spiders. Upon taking enough damage, it separates into a head and body. The head slides towards Isaac, leaving a trail of damaging red creep. The body wanders around aimlessly, occasionally spawning spiders.
The Mask is invulnerable and moves around erratically, charging at Isaac when he lines up with it. The Heart avoids Isaac and fires 4 blood shots in the cardinal directions when Isaac gets near. Killing the Heart will automatically kill the Mask.
Falls from the ceiling. If it lands on Isaac, it deals damage and teleports the player back to the spawn room of the floor. If it doesn't land on Isaac, it spawns several rock waves that travel outwards. Spawns several spiders upon death. Entering a room containing Mom's Dead Hand will warn the player with Mom's laugh.
Stationary enemy that is normally invulnerable. Raises its head up when Isaac gets close, revealing a fleshy stem that can be damaged. Upon raising its head, it will fire 3 blood shots and lower its head again.
When Isaac lines up with it horizontally, it charges in his direction and then doubles back, charging back to its original position. When Isaac is not horizontal to it, it spawns Kamikaze Leeches. Explodes when it dies.
Some monsters can spawn as a Champion which deviates in coloration and size from its more common form. The champion's coloration denotes which peculiar behaviors and traits it will exhibit. Champions will generally drop a pickup (coinciding with the color of the champion) when killed on normal difficulty. However, in hard mode, fewer champions will drop pickups on death. All champions will deal contact damage, regardless of what monster they are (e.g. a common Mulligan will not deal contact damage, but champion versions will). Some champions inflict full heart damage regardless of chapter. The likelihood of champions appearing is increased by unlocking the Everything Is Terrible! achievement, by carrying the Purple Heart trinket, by picking up the Champion Belt item, or by playing in hard mode. Champions are generally regarded as more dangerous than common monsters.
The identifying scheme for champion qualities is as follows: