Pacers typically wander around blindly in an area, and have a low amount of health. This makes them of little threat to the player.
Note : 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).
Gushers wander around a room blindly, similar to Pacers, but will occasionally shoot blood in a random direction. The shot is slow-moving and has short range, and is typically easy to avoid. Gushers are only a threat in large numbers and enclosed areas.