All Cybertronians can transform into any kind of vehicle, weapon, animal, etc. Whatever they want on Cybertron or on Earth. A transformer's Transformation Cog enables them to physically transform. Without it, they wouldn't be able to do this.
The original Thirteen all possessed the ability to transform but when the Cybertronian race was born, Alpha Trion overrode Prima's desire to inform the newborn species about their potential for transformation. The rogue creations of Quintus Prime, the Quintessons, were aware of this dormant power. Upon landing on Cybertron, they "bestowed" this gift upon the awed Cybertronian race, claiming it was part of the Quintessons' benevolent intentions to "uplift" the Cybertronian race, when in reality, they were remotely activating the T-Cog. As the rule of the Quintessons grew crueler and more decadent, the resistance intercepted the newborn Silverbolt from the Well of All Sparks with an examination of him revealing that the T-Cog, and thus the ability to transform, was a natural part of Cybertronian biology.
The legendary T-Cog of Amalgamous Prime was eventually found in orbit of a black hole by a group of refugees fleeing the destruction of Neutronia. Exposure to the energies of the artifact gave the colonists the potential for infinite transformation and they rebranded themselves as the Mutacons.
After emerging from the Well of All Sparks, newborn Cybertronians had to learn how to transform. Exodus A rare sub-species of Cybertronians, the Shifters, were able to transform into others of their kind, an ability which made them highly valued as spies.
After reformatting the Lightning Strike Coalition Force with alien alternate modes, Shockwave deliberately barred Grimlock's ability to transform, wanting the Autobot to figure that he could now transform only when sufficiently angry. The other members of the group still retained their full transformation potential however.
After MECH had determined the purpose of the T-Cog, they baited a trap for any Cybertronian and harvested Bumblebee's T-Cog, depriving him of his ability to transform or even access his internal weapons. After the scout reclaimed his T-Cog, MECH harvested Starscream's leaving the Seeker stuck in his robot mode.
After cloning himself, Starscream was shocked to find his CNA had provided the clones with working T-Cogs and transformation abilities.
After witnessing several Vehicons transforming, Predaking began to wonder if he too could transform and access a robot mode. He could. This was little surprise to Shockwave who knew transformation to a be a fundamental aspect of Cybertronian biology. Nonetheless this was enough for Megatron to terminate Project: Predacon in the fear the beasts would turn on him. Evolution Optimus Prime was equally troubled by Predaking's transformation.