Transmetal

A Transmetal is a (generally) Vok related enhanced form of a Beast Wars era Transformer, featuring a "mutated superstructure".

Normal Transmetal
In beast mode, the Transmetals seemed almost, if not entirely, mechanical, much like the animal Transformers of the previous Generation 1 franchise. However, as part of a partial "inversion", many of their robot modes bore organic elements, such as Cheetor's furry chest. Transmetals also typically featured a third "mobility" mode. While not full vehicular modes, these half-modes gave aspects of a vehicle to the existing beast forms. Examples included Rattrap as a wheeled rat and Megatron as a jet-powered inline skating enthusiast.

Beast Wars animated series
The Transmetals were created as a result of transformers being bathed in a quantum surge after the destruction of the Vok Planet Buster. All the Beast warriors who were not "born" from a protoform on Earth or shielded in a CR Chamber were altered. (Why Earth-"born" Transformers were immune to the surge's effects is unknown.)

The affected characters were Megatron, Tarantulas, Rattrap, and Cheetor. Scorponok and Terrorsaur were in the middle of their mutation when they fell into lava pits beneath the Darkside and perished. Optimus Primal, Rampage, and Depth Charge also eventually became Transmetals as a result of the surge.

Most Transmetals had additional weapons and seemingly enhanced power levels. They were immune to many Vok weapons, and may even be immune to the effects of energon radiation.

This last point had little bearing on the plot, since most of the energon deposits on Earth's surface had been destroyed by the Planet Buster cannon. However, in Tangled Web, Tarantulas is standing beneath an energon cache, completely unaffected. Meanwhile, in The Agenda, the non-Transmetal Inferno suffers energon overload while attempting to kill Optimus Primal and Cheetor. So immunity to the radiation may be a permanent effect of the Transmetal body.

According to Ben Yee, the Transmetals were originally conceived of by Beast Wars writers as "absorbing" energon radiation and then needing to later discharge that energy, typically as weapon energy. However, the only time this was hinted at in the series was during "Bad Spark", when Cheetor "farts" a little energon cloud.

The Fuzors Silverbolt and Quickstrike, like the Transmetals, were shown to be immune to Vok technology, seemingly possessing qualities of Transmetals.

Toys
In addition to the characters mentioned above, Transmetal toys were also produced for Rhinox, Airazor, Waspinator, Terrorsaur, and technically Inferno (the Scavenger toy was initially intended to be his TM form, and was so in Japan and the UK.) None of these appeared on the show.

Scorponok, Blackarachnia, and Dinobot, though also not Transmetal in the show, were given their own Transmetal toys as Happy Meal pack-ins from McDonald's. Claw Jaw and Spittor were redecoed and sold as Transmetals in European markets with a VHS tape.

Transmetal 2
Near the end of the series, a new breed, known as Transmetal 2s appeared. These new forms blurred the line between the mechanical and the organic even further, with equal parts of both appearing in both modes in an asymmetrical patchwork like jumble. This often resulted in somewhat monstrous looking designs.

Beast Wars animated series
Within the context of the television series, the idea of a Transmetal 2 was somewhat vague. Only Cheetor, Dinobot 2, and Blackarachnia were definitely meant to be Transmetals 2. Each of them was altered by an enigmatic piece of Vok technology known as the "Transmetal Driver" which appeared early in season three. The exact origin and nature of the Driver is unknown.

Optimus Primal and Megatron also received new bodies in season three, in the same fashion as each other, but only Megatron's toy was labeled as a Transmetal 2. (Optimal Optimus was a normal Transmetal.) It is unclear whether, within the story of the cartoon, these bodies were meant to be similar to the bodies granted by the TM Driver. Both of their new bodies were the result of temporarily merging their sparks with the sparks of their Generation 1-era counterparts, Optimus Prime and G1 Megatron.

Tigerhawk's toy was also labeled as Transmetal 2. His in-show origin involved both the fusing of two sparks, those of Tigatron and Airazor, and Vok technology.

The new forms for all of these characters, whether produced by the Transmetal Driver or by spark fusion, were portrayed as substantially more powerful than normal Transmetals.

Toys
According to their toy packages and bios, the Transmetals 2 were all the results of Megatron's cloning experiments. This roughly aligned with the beginning of the cartoon story, as the first Transmetal 2, Dinobot 2, was created by Megatron. This cloning experiment angle also helped to explain the presence of Optimus Minor.

Unlike Transmetals, most Transmetals 2 did not have a third mode. Only the largest Transmetal 2 toys, Tigerhawk and Megatron, were designed to have more than two modes, but Megatron's was never featured in the TV show and Tigerhawk's was dropped entirely even from the toy. Neither vehicle mode was especially well-implemented.

Early on in the release of TM2 toys, the packaging used a Roman numeral "II", but a running change in the packaging was made to the Arabic number "2".

IDW Beast Wars comic
In the IDW Beast Wars comics, a new group of Predacons traveled back to the Beast Wars era (taking place in the third season of the program around the episode "Changing of the Guard") and activated the remainder of the stasis pods ejected from the Axalon. The Transformers who emerged from these pods included normal beast-form Transformers, Transmetals, and Transmetals 2.

All the Beast Wars characters/toys not featured in the original cartoon (barring a few that traveled back with Magmatron) were given this same origin regardless of their "Transmetal" status. It was not explained why their states differed, nor why their situation differed from the non-Transmetal Earth-"born" Transformers in the show.