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."

Transmetal 2

Near the end of the series, a new breed, known as Transmetal 2 appeared. These new forms blurred the line between mechanical and organic further, with equal parts of both appearing in both modes in an asymmetrical patchwork. This often resulted in a somewhat monstrous looking design. The specific process by which each Transformer became a Transmetal 2 varied. Cheetor, Dinobot (2), and Blackarachnia all were altered by what was known as the "Transmetal Driver", an enigmatic piece of Vok technology apparently altered by Megatron to suit his own purposes. Megatron's form was altered when he merged his spark with that of G1 Megatron's, similar to Optimal Optimus. Tigerhawk was apparently built as-is by the Vok. No other Transmetal 2s appeared in the Beast Wars series, though a number of other toys were produced. These new forms were all considered to be substantially more powerful than a normal Transmetal's. Unlike Transmetals, most Transmetal 2s did not have a third mode. Tigerhawk and Megatron, both higher price-point toys, did have third modes, but they are not considered by most to be very well implemented, and these were not used in the show.

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