Buu sans is character that goes by many names due to translation issues with the anime and manga. He goes by Buu sans, Sans Buu, and even Majin Sans. Even with all these names, Sans Buu is his canonical name.
Buu Sans is just Majin Buu/Super Buu except instead of absorbing Gohan he instead aborbs a stronger being that is known as sans from the hit indie-game, UNDERTALE. With this new power, Buu is stronger than ever before. When Goku and Gohan fuse into Gokhan, all Buu sans does is constantly mock the two as he constantly dodges with no fatigue and counters each hit with either a Gaster Blaster or even Bones.
Sans Buu was awakened on October 7th, 2019 on Twitter when one person thought that editing an image into sans would not bring any attention towards it, they were wrong.