In G1 canon, Pretenders are Transformers that have a sort of shell that goes around their form, granting them additional abilities or weapons. The robot inside is entirely hidden within the shell, though can emerge at will. The word Pretender can refer to either the shell itself or any Transformer that has one.
At 2k5, a Pretender TP was run around 2029 IC year in which Thunderwing and some of his cronies created Pretender shells that any Transformer could use. These powerful enhancements were eventually gained by both the Autobots and Decepticons, but were revealed to be part of a grander scheme by Thunderwing. Eventually the shells were destroyed, with the exception of two: the "Eagle" suit remained in the possession of the Autobots, and the "Octopus" suit is kept in cold storage by the Decepticons.
Pretender technology had resurfaced in one of Thunderwing's secret laboratories in the form of hideous half-machine monsters (one of which strongly resembled the original octopus suit).
In the year 2031, Thunderwing was back at it again.. only this time perfecting his Pretender technology. And so he set out to abduct random 'participants', more like test subjects, who fixed his predetermined profile. After having absconded with them, he forcibly merged them with his Pretender technology.. granting them access to a shell, but kept in check by his mind control. It was later revealed the big plan was to utilize the Pretenders to gain access to the 12th and final lense.. therefore giving him the power of Hytherion. Only Bumblebee, aided by Loadout, ended that scenario.. and the respective factions found a way around the Pretender programming.
Pretenders that took part in the TP were introduced, but any potential Pretenders (Unlisted FC Policy) are to have been there the whole time behind the scenes.