The Ancient Decepticons are a group of Decepticons who came Earth in the distant past and disguised themselves as monsters. They were imprisoned by Autobots Evac and Crosswise, but freed by Starscream and converted into his personal army.
The Ancient Decepticons, were originally stranded on Earth after coming there on the starship Atlantis. They also left several artifacts that were later discovered by Dr. Lucy Suzuki.
They also somehow figured out how to transform their sparks into looking like monsters. They terrified humans in these forms and inspired many monster myths and legends. However, they were all eventually hunted down by Evac and Crosswise, who imprisoned them in stasis pods which they stored in the North Pole.
However, thousands of years later, they were all freed by Starscream, who recruited them all as his army. Starscream ordered the Ancient Decepticons to find the Earth Cyber Planet Key. Many of the Decepticons merged into Seeker Ships and circled the globe in search of the key. The Autobots attempted to oppose them, but failed.
After Starscream acquired the Omega Lock and the Cyber Planet Keys, the Ancient Decepticons retreated to Starscream's volcanic lair and watched as Starscream harnessed and absorbed the Lock's power. However, the Autobots showed up to reclaim the Key and the Ancient Decepticons fought the Autobots while Starscream absorbed the rest of the Lock's power.
Eventually, most of them were defeated and re-sealed by the Autobots, with Starscream retreating alongside Sideways. The only Earth Decepticons to avoid capture were Thunderblast, who joined Megatron's faction of Decepticons, and Lugnutz, who was left wandering around wondering where Starscream and the others had gone to.
The Ancient Decepticons were later freed again, this time by the Autobots, to aid them in the fight against Galvatron. In exchange for their help, the Decepticons were given freedom, and they later aided the rest of the Transformers in their space bridge project.