Sweet Little Dead Bunny
Sweet Little Dead Bunny is the last song you'll hear in Doom. When you've finally made it through all three terrifying episodes, you'll be rewarded with a portal back to Earth. There, you'll see a happy bunny grazing in the field, and, for the the first time since you started playing, a happy upbeat tune. Unfortunately, the music soon turns sinister as the demons of Hell follow you through the portal and begin invading the Earth, killing the fuzzy bunny in the process.
Its internal name is "D_BUNNY" within DOOM.WAD file. This track is not included in the shareware version of the game.
Sweet Little Dead Bunny is featured in the Sega 32X game and plays when you complete the game. However, there is no any gruesome bunny scene here but credits. Of all versions, this one can be thought of the worst: after a good beginning of the happy vibe the sinister guitar part sounds exceedingly terrible.
Most of the listeners consider the Doom soundtrack arrangement on the 32X port the worst one.
The song Sweet Little Dead Bunny is featured in the 3DO but is never played in-game. The lack of ending sequence explains missing such opportunity.
Final Doom: The Plutonia Experiment (DOS)
Sweet Little Dead Bunny is featured in Plutonia Experiment expansion of Doom II, but rather than being used in the bunny scene (which is not here) or any special win screen the song is used in a normal level. It plays in the Level 11: Hunted, the final of the first episode, populated by Arch Viles having nothing to do with bunnies. The tune does loop in the game, therefore getting on player's nerves because the song is too short to quickly annoy after the third or the fourth loop.
The music track's internal name is "D_STLKS2" within PLUTONIA.WAD file.