The Unon, also known as the Riders, were a centaur-like species who hailed from the planet Zentauron (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) who came to prominence after the Last Great Time War, to heal the damage caused to the universe by the Time Lords.
Initially explorers and scientists, the Unon barely survived the Time War, and became a race of crusaders, led by the mother superior Arnora, intent on healing time fissures before they spread across space. They also came into possession of an entropy engine which survived the Time War. Using it, and scavenged samples of Time Lord technology, they erected the "Perpetual City", and used it to destroy planets, stopping other species from developing time travel. One of these civilisations was Excroth, the planet of the Excrothians. The Excrothians used the very same time travel experiments that brought the Unon to them to escape, and they placed themselves inside tanks and began calling themselves the Lect, who sought revenge.
The Lect and Unon fought one another on Fluren's World, when the planet's emergency protocol caused the time bubble protecting the planet to vanish. The Unon brought the Ninth Doctor and Captain Jack Harkness to the Perpetual City, and the Doctor was tricked into using a temporal stabiliser to heal a time fissure on the planet Traxis. The Lect brought the Doctor to their ship and revealed they planned to destroy the entropy machine. Prelon Marleth turned the engine on itself, and he and Arnora destroyed one another. The Doctor took the Unon in the TARDIS to another planet to start over again, and let the universe look after itself. (COMIC: Weapons of Past Destruction)
Their activities, particularly their repurposing of Dalek technology, were detected by the New Dalek Paradigm who dismissed them as "upstarts" that they were content to allow the Doctor to handle as they focused on building their empire. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)