The Pretenders are a subline in the Transformers universe introduced in 1988. The concept was that the Transformers were capable of disguising their robotic forms through the use of synthetic organic outer shells.
Within the Transformers universe, disguise has always been a key component of Autobot and Decepticon operations on other worlds. Pretenders attempt to take this concept to the next level: They are Transformers fitted with special external armor shells that can disguise their very nature as robots. Rather than hiding themselves as the machinery or vehicles of a world, Pretender Shells allow a robot to take on the appearance of a native life form itself.
On top of its exploration and infiltration uses, a shell can also function as powerful battle armor, aid in self-repair, and even function independently of its robot as a second combatant mentally controlled by its "wearer". The major drawback of the technology is that the core robot will feel whatever pain is inflicted on the shell, sometimes even more strongly than pain inflicted on the robot itself.
The most common style of Pretender is an inner robot with a single humanoid shell (usually resembling a monster for Decepticons (except in the Surgeon stories where the main Decepticons like Starscream are more monstrous) or a human in battle armor for Autobots)
The Surgeon stories:
Along with the Decepticons, the Pretenders are the only other life form on the parallel world encountered by the Surgeon and his companions that are truly shown free (the Slitheen are also free but are shown to be more secretive and underground). And like the Slitheen, the Pretenders take on the form of various comic book characters from the Marvel and DC Universes (just for the sake of clarity because getting everything on this wiki for the Pretenders would take forever). Unlike the Slitheen however, who take on the form of villains (including some that the Surgeon had met in the past such as The Joker and The Riddler), the Pretenders take on the form of heroes and villains.
They also take on the forms of the "desirables" that were put into concentration camps by the Decepticons; at least, the desirable survivors which largely consist of women which means that a), these Pretenders are the second Amazon brigade to exist on this world with the first being the one led by Slipstream consisting of female Decepticons and Autobots, the latter of whom are forced into the Decepticon Army after they take over and b) the Decepticons may have captured men and children to put into concentration camps, but once the Pretenders scan all the women, these are disposed of, though the forms taken on by the Pretenders below are freed and sheltered by the Slitheen.
Pretenders also exist further back before the Surgeon even comes to the Earth in Revelation because there is a Pretender who was captured and tortured by Doctor Doom (who later reveals he had destroyed it, so the Surgeon has indeed made a part of the Decepticons extinct because that was the last male Pretender implying that others were destroyed by the Autobots on the parallel world). The Pretenders are eventually destroyed by the very prisoners they took the forms of, while the Surgeon, his companions and the Slitheen deal with the Decepticons and Doctor Doom and Brainiac deal with The Fallen.
Forms taken on by the Pretenders in the parallel world
Also used as Slitheen disguises: