Greek Πύθων was a mythical monster slain by Apollo. There was another monster named Τυφων, and according to Watkins these words were doublets in Proto-Indo-European - *b(ʰ)ud(ʰ)-n- and *dʰub(ʰ)-n- - referring to "bottom," "foundation," "depths," and the monsters that lived there. They were from *dʰeub- (also *dʰeubʰ-) "deep, hollow". Greek Πύθων was borrowed into Classical Latin as Pȳthōn, and eventually used for a kind of snake.
In Proto-Celtic *dʰeub- became *dubno- "world" from the sense of "ground" or "bottom". The form *dubno-walos "ruler of the world" became the Old Irish name Domnall, the source of English Donald (also from Watkins).