Transmetal

Transmetals are a mutated, and greatly enhanced, form of Transformer who appeared in the Beast Wars series.

As a result of being basked in an alien Transwarp Wave, several Transformers who were not born on earth, or in the Cr-Chambers/Baths at the time(Cheetor, Rattrap, Megatron, Tarantulas, and, later, Optimus Primal, Rampage, and Depth Charge) became Transmetals.

In beast mode, a Transmetal is more superficially mechanical then his or her previous beast incarnation, and the Transmetals typically have an extra "mobility" form, such as Cheetor's jets or Airazor's pontoons. Also, most Transmetals have additional weapons and enhanced strength. To go even further, Transmetals are immune to many Vok weapons, and possibly immune to the effects of Energon Crystals as well (this last point had little bearing on the plot, since most of the Energon had been destroyed by the Planet Smasher cannon).

A peculiar aspect of the Transmetallization process was the "inversion" of the Transformer's body. While the beast mode became almost wholly mechanical, the robot mode bore some organic elements, particularly Cheetor's furry chest, and Rattrap's exposed "brain."

Near the end of the series, a new breed, known as Transmetal 2 appeared. The specifics of how this form was reached changed from Transformer to Transformer (mostly by exposure to Vok artifacts). The Transmetal 2's had a completely different appearence then their previous forms and were even stronger still! However, Transmetal 2s did not have a third mode. In the toyline, some Transmetal 2s, like Tigerhawk and Megatron, were presumably designed as Transmetals as they had third modes.

From Teletraan I: The Transformers Wiki, a Wikia wiki.