Here is a list of cheat codes that can be found by playing the game.
Swamp thing: 20-18-2-22
Can be found through the tunnel outside in the dog head area. The roman numerals are the code.
Turns the bloody monster into a green monster.
Pretty Princess: 1-9-90
Can be found in one of the first four levels, the bloody hallway one. After killing the heart, take the right most path back to the circular room. A tombstone is there with the code on it.
Turns the bloody monster into a pink monster.
Super Sprint: 19-30-9-17
Can be found by completing the game and running all the way through the credits section.
Increases your run speed.