Mercurials were native to the planet Mercury.
Mercurials were tall, humanoid, and smooth and silver. Their arms ended in points and their feet ended in flat discs.
Soon after the Sunwatcher project was set up on Mercury by Earth humans, the Mercurials were attacked by "giants". One of the Mercurials was found and brought onto the base, and it tried to communicate with the humans to warn them of the threat. With the help of the Second Doctor, the Mercurials learned that the "giants" were actually the solar pylons that powered the base. (PROSE: Mercury)