The Sidhe were a species native to Earth.
Although the Sidhe were native to Earth, they lived out of phase with human perception. The Sidhe existed in, and were able to perceive, all eleven dimensions. They could appear humanoid to human perceptions, with vaguely cat-like features with wide cheekbones, a pointed chin, pointed ears, and a flattened skull.
The Sidhe could make humans vanish into their realm. They were essentially immortal, and could move through time. They were vulnerable to iron, due to their electromagnetism-based lifestyle. (PROSE: Autumn Mist)
The Sidhe had various tribes and clans, including the Leannain Sidhe. The Leannain Sidhe called the Doctor the "Evergreen Man". They pay homage to the forces of order and chaos, with their queen Titania, personifying order and Oberon personifying chaos. (PROSE: Autumn Mist)
The Sidhe inspired the Leshy of Polish folklore, the Kachinas of Hopi mythology, elves and fairies among other mythical beings. The Sidhe formed a truce with mankind long ago, after Sidhe and humans fought a war against each other. The Sidhe were aware of the Celestis.