Mysterious Pond

The Mysterious Pond is a location from The Legend of Zelda: A Link to the Past. Located near the base of the Pyramid of Power in the Dark World, Link can gain access to the Mysterious Pond by using a Super Bomb to blow up the wall that is preventing entry. Once inside, he finds a small chamber containing a pond, similar to the other Great Fairy Fountains in the game.

Within the Mysterious Pond, Link meets a Great Fairy who is morbidly obese. She respects the hero's honest personality and grants him the Silver Arrows that are needed to defeat Ganon. She will also turn Link's Master Sword or Tempered Sword into the ultimate Golden Sword.

Theory
It is unknown why the Great Fairy residing in the Mysterious Pond is depicted so differentally from the other ones, but there are several possible reasons. For one, her depiction could purely be comic relief, meant to make the player laugh. Another option is that the Evil King could have turned her into that, due to her being so close to his domain, or because she is in the Dark World. A final idea is that it could not be intentional at all, and the developers were just trying something new.