Pacers typically wander around blindly in an area, and have a low amount of health. This makes them of little threat to the player.
- They can also be spawned by Mr. Maws and Gapers. They are also spawned by Mom during the boss fight.
- A champion Pacer will maybe spawn in rooms that there is already Pacers (by replacing the normal one by a champion one) or by killing a champion Gaper / Frowning Gaper / Mr. Maw (killing a normal one will result in a normal Pacer or, in Mr. Maw's case, a Maw can spawn).