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 (Cheetor, Rattrap, Megatron, Tarantulas, and, later, Optimus Primal and Rampage) became Transmetals.

In appearance, a Transmetal is more mechanical then their former Beast counterpart, and they typically have something of an extra alternate form. 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).

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 a Vok artifacts). The Transmetal 2's had a completely different appearence then their previous forms and were even stronger still!

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