I can imagine Onionsan being a kraken type thing that terrorises waterfall, and either he has his own territory that people avoid or Undyne killed him in some awsome, gruesome fight.

Onion-san hunts the depths of the Dead Swamp, his tentacles sweeping the paths in search of daring prey. No one has seen his face since the Hunger took true hold. Either he can no longer breathe pure oxygen, or he is too ashamed of what he has become to show himself.