After beating a world, most of them will reward you with a bonus stage. These bonus stages are Pokemon themed with the background having various Pokemon objects and songs from the franchise as well (Route 26, Champion, etc.)
There are 5 stages in total:
- Pokemon Gym: Unlocked after clearing Marching Hills
- Route 26: Unlocked after clearing Rock Range
- Bicycle (From Gold & Silver): Unlocked after clearing Calypso Beach
- Champion (From Red & Green): Unlocked after clearing Sleigh-bell Slopes
- Trainer (From Black & White): Unlocked after clearing Baroque Volcano
- This was most likely implemented due to Game Freak's most successful franchise, Pokemon, since most people recognized them for it.
- All the pictures you unlock after beating the Fast Modes are the facial studies of the main characters beside the final one, which is a crossover picture of Pokemon and Harmoknight.
- There is a subtle reference in Pokemon X and Y to HarmoKnight where a NPC will mention the game.
- For every stage,there will be a cameo by a pokemon.In Pokemon gym,balloons with the face of Pikachu will appear.In Route 26,balloons in the shape of a Tepig will appear.In Bicycle,rock statues of Oshawott appear.For Champion, ice sculptures of Vanillite appear,while in Trainer,statues of Reshiram and maybe Zekrom can been seen.