Once, they were just a breed of mechanical vermin, But the Unicron mutated them, and under the command of the power-hungry sentient Scrapmetal, they swarmed the surface of Cybertron. It is believed they may even be descendants of the Insecticons.
Major Story Arcs
Cybertron: Balancing Act
As the Scrapmetals swarmed over the abandoned Cybertron underneath the Unicron Singularity, several menaced the new arrival Skyfall before being destroyed by Dark Scorponok. Other Scrapmetals were co-opted by Nemesis Prime and Ramjet in the service of Unicron.
The Scrapmetals followed Ramjet into battle when he confronted Skyfall. Ramjet used the Scrapmetals to attack Primus's spark core chamber. Although Ramjet succeeded, the chamber's defender Downshift destroyed the Scrapmetals.
Powers & Weaknesses
Individual Scrapmetal have only a base animal-level intelligence, and by Cybertron standards are small and weak, easily crushed. They are only powerful in swarms, when they can overwhelm even the largest Transformer through sheer strength of numbers. They appear to follow the orders of a single "barely" sentient Scrapmetal.
Transformers: Cybertron (RID) (2005)
The Transformers know about Scrapmetal as a planet-hopping scourge.