The Toclafane were fairy tale monsters from Gallifrey, their name, like the Bogeymen on Earth, just made up.
In 2008, the Master claimed to have contacted the Toclafane (in reality a race of Human-based cyborgs) and, using a paradox machine linked to the Doctor's TARDIS, allowed the entire race to pour through to decimate the Human population and create a Time Lord Empire. (DW: The Sound of Drums / Last of the Time Lords)
Behind the Scenes
The Toclafane is based on the Disney character, Toclafane who looks similar although is a lot more friendly.