The original Megalovania it plays when you square off against Dr. Andonuts
Keep in mind that this is from a rom hack made by Toby Fox, and it isn’t in any of the vanilla mother games
Bonus: It’s not like this was an mp3 Toby made and slotted into the game’s files. Earthbound (and N64 and previous games in general) had their music generated from tones being synthesized in the game itself to save on space. Toby got into the guts of Earthbound, programmed a whole song himself using backwards modding tools of an SNES game, and got the catchy tune we all meme today out of it.