Monstrons, also spelt Monstroms, (COMIC: Weapons of Past Destruction) were a race of blue-skinned humanoid chrononauts. (COMIC: The Menace of the Monstrons, AUDIO: The Iris Wildthyme Appreciation Society) They had robotic servants named Engibrains, and they claimed to have a President.
Two Monstrons, Kenex and Yeq, landed on Skaro and made their base inside an extinct volcano. Their attack succeeded in destroying the Dalek City, though the Emperor Dalek and others escaped underground. Their attempt to conquer the planet was defeated by the Dalek which they had captured, which escaped and sacrificed its life to set off a volcanic eruption that destroyed the Monstrons. (COMIC: The Menace of the Monstrons)
Despite their seeming banishment from N-Space, a Monstron Time destroyer once pursued Iris Wildthyme's Celestial Omnibus across time. (AUDIO: The Iris Wildthyme Appreciation Society, Iris Rides Out, Midwinter Murders)
Whilst Jack Harkness was working for the Time Agency, he visited the Fluren Temporal Bazaar to bid on a Monstrom time destroyer. He lost the bid to a pair of Wrightosaur mercenaries. (COMIC: Weapons of Past Destruction)