Primus
Primus is, in almost every single continuity, the creator and very living God of all Transformers.
His specific origins vary from incarnation-to-incarnation, but like Unicron, there are many elements which are similar.
Primus is an incalcuably ancient being (almost always the "God of Creation") who battled the Chaos Bringer, Unicron, at the dawn of time. Despite his vast power, Primus was incapable of defeating Unicron, so he outsmarted his foe, trapping them both into large planetoids.
Over time, both Primus and Unicron learned to shape their prisons from within. Unicron used form to become an immense robotic planet which had the ability to change into an immitation of his former self. Primus, in turn, became the mechanical planet of Cybertron. Knowing that his ancient enemy still lived, Primus created a new life-form from the newly created planet. A mechanical creature capable of changing forms, the very first Transformer. To this Transformer, Primus bestowed the Creation Matrix, a tool which contained the very living essence of Primus.
Marvel Comics Continuity
Beast Wars/Beast Machines
Universe
When Unicron began gathering Transformers from the multiverse, forced them to fight, and harvesting the Sparks of the losers, this presented a huge threat to Primus and the Allspark. Transformers' Sparks are pieces of Primus' essence, so each devoured soul weakened him.
Primus and Alpha Trion called back from the Allspark the now-legendary Maximal ship commander and Cybertron's savior, Optimus Primal, to rescue the possessed, battling captives from the Pit of Unicron -- and to lead a team of multiversal warriors to put a stop to his soul-stealing scheme before it could cause the Allspark to collapse.
(This storyline was never completed.)
Unicron Trilogy
Energon
Cybertron
