Toby Fox's "Megalovania" was first used where?

The answer is not "Radiation's Earthbound Halloween Hack." Megalovania was actually first used in the game "Touhou Mother," which is a fan-made crossover between the Touhou Project and EarthBound/Mother series. Toby Fox, the composer of Megalovania, created a remix of the song "Megalomania" from EarthBound, and it was featured in this fan game. Later on, Toby Fox re-used the song in his critically acclaimed game "Undertale," where it gained widespread popularity.