Fatass is a tracker riten by Jeremy Evers. It is a stripped down FastTracker II clone for the GBC. The difference between the 2 is that FastTracker II has effects columns wile fatass just has a note column for each of the 4 gameboys sound channels, those can be manipulated via the instrument editor for duty, waveform, panning, vibrato, arpeggio, portamento, sample playback and all of its hardware features.
The special thing about Fatass is that its sound engine uses a method of linear frequency handling, that means no-madder what note you play, it'll still be on a musical scale.
Fatass also comes with several songs bilt-in to the ROM, some of the songs may sound pretty familiar as this was the tracker that was used in a not so playable game for the GBC called Animorphs. Not all songs from the game are included though, just some.

Fatass can be downloaded at Jeremy Evers official website