In-game legends state that the land was created by the Four Giants. In the official Hyrule Encyclopedia, it is stated that Termina is nothing more than a figment of the Skull Kid's imagination brought to life by the mask, and that once the mask was defeated and Link left, Termina ceased to exist.

Was Termina created by the mask?

The answer it reveals is that Termina is merely a fabrication, created by the power of Majora's Mask when worn by the Skull Kid, and the world ceases to exist once Link departs at the end of the game.

Who created Termina Zelda?

Trivia. According to The Great Hyrule Encyclopedia formerly hosted on Zelda.com, Termina is one of millions of alternate worlds that was created as an unanticipated side effect of the Creation of the World by the three Golden Goddesses.