Minish
The Minish or (known as Picori by the rest of Hyrule) are a small race of people no bigger than Link's thumb. A long time ago, the Minish gave the Hero of Men a sword of unparalleled power; the Picori Blade. They also left behind the Light Force for the Royal Family. Their help is celebrated annually in Hyrule through the Picori Festival. Every one hundred years, the doorway between their world and the world of men appears in the courtyard of Hyrule Castle. That doorway is also the entrance to the Elemental Sanctuary. They have only appeared in The Minish Cap. Vaati was once a Minish too.
Quote from The Legend of Zelda: The Minish Cap Forest Minish figurine:
"Not visible to the eyes of adults, they delight in making humans happy by hiding helpful items and Rupees under grass and rocks all over the world." (This may explain why, throughout the Zelda series, items can be found by slashing grass and throwing rocks.)
Quote from The Legend of Zelda: The Minish Cap Mountain Minish figurine:
"These seven students followed Melari from Minish Village to Mount Crenel. Their song is actually a sign that they are full-fledged Mountain Minish now."