Pretenders hide the Transformer inside!
A Pretender is a Transformers toy which has an outer "shell", disguising the transforming robot. In most cases, the outer shell is an armored human (in the Autobots' case) or as an armed monster (Decepticons). In addition, there are two Pretender Beasts on each side where the outer shell is an armed animal, and one Pretender Vehicle on each side where the shell is an armored vehicle.
The main downside to the Pretender toys was their size. The outer shells were designed to be as big as a large action figure, but to make them sturdy enough for play, there was a lot of wasted space between the shell and the robot inside. The inner Transformer was inevitably skinny to the point of anorexia, and their simplistic vehicle modes were forced to conform to their robot modes rather than the other way around.
The Pretenders, like the Powermasters, were introduced as toys after the American cartoon ended. In the Transformers comics (issue #40), the Pretenders were a "synthoplasmic" shell created by Scorponok/Zarak to disguise Decepticons and hide them among humans. The technology was intercepted and duplicated by the Autobots shortly thereafter.
In the Japanese "Transformers: Masterforce" cartoon, the Pretenders were introduced without origin in the first episode "Arise, Pretenders!" Their shells not only disguised their robot forms, but shrank them down to human size, so that the Autobot pretenders could actually impersonate humans. This paralleled the Godmaster:Godmasters and Junior Headmasters, who were giant robots completely controlled by human "pilots".