Time Riders rode the time winds of the Vortex. They travelled in packs.
A Time Rider named Vorac was separated from his pack and was picked up by the Doctor's TARDIS. He was rescued by Jamie McCrimmon and Zoe Heriot, and, according to the code of the Time Riders, the Second Doctor was taken from the TARDIS until Vorac could be returned to his people. (PROSE: Undercurrents)