From the creation of mass intellect came the Combiners and all their subsequent incarnations.
For centuries, the Transformers worked on the project of mass intellect, the fusing of multiple minds and bodies. However, all those experiments were failures, as the electro-neural meshing was hampered by the presence of strong minds without drowning any weaker ones out. During the Great War, the Constructicons finally succeeded where others had failed, stabilizing the electro-neural meshing with a hyper-processor vortex. They tested the theory on themselves, and the result was Devastator. While a success, the mass intellect was hampered by a need for Devastator's components to agree on a course of action. This difficulty caused the combined form to suffer from a "dumbing down" problem. Although it could be considered a failure, Megatron was pleased with Devastator's power, and commissioned further studies into mass intellect.
However, the Autobots eventually got a hold of the mass intellect blueprints, and the Autobot science harmonium drastically overhauled the process, making a more efficient Combiner, resulting in the Special Teams of the Aerialbots and the Protectobots, and creating Superion and Defensor. The Decepticons countered this by having the Stunticons and the Combaticons undergo the mass intellect process. Even with the more refined process, the Autobot combined forms still suffered from mental difficulties. The Protectobots in particular were trying to iron out the drawbacks while Optimus Prime and Megatron were missing and the Special Teams were declared non-aligned.