Nezumiman
Nezumiman is a parody/tribute to the NES game, Mega Man. This cool 2d shooter platformer lets you play as a rat that can do almost anything Megaman can do (including the infamous sliding maneuver that became prominent since Mega Man 3.)
The game is quite simple to play unlike Mega Man, but it is fun. Your goal is to defeat the evil, Dr. Gyoniku and a his gang of evil rat-like creatures. There are 8 levels in total and you have to beat every single boss in the level to advance.
Almost every single aspect is similar to the blue robot series, the only marked difference is the graphics and instead of fighting big bad weird robots, you just fight off weird animals. Pretty cool game that's worth your time.