Transmetal
Transmetals are an enhanced form of Transformer with a "mutated superstructure" which appeared in the Beast Wars franchise.
Normal Transmetal
In beast mode, the Transmetals were almost entirely mechancial, much like the animal Transformers of other TF franchises. In addition, as part of a partial "inversion", many of their robot modes bore some organic elements, such as Cheetor's furry chest. Transmetals also typically feature a third "mobility" mode. Examples include Rattrap as a wheeled rat and Megatron as a jet-powered inline skating enthusiast.
Beast Wars cartoon continuity
As a result of being basked in a quantum surge after the destruction of the Vok Planet Buster, all the Beast Warriors were mutated into Transmetals except those who were "born" from protoforms on Earth or were sheltered in CR Chambers/Baths at the time of the surge.
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. Later, Optimus Primal, Rampage, and Depth Charge were also mutated into Transmetals.
Most Transmetals have additional weapons and seemingly enhanced power levels. They are are 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.)
Toys
In addition to the characters mentioned above, Transmetal toys were also produced for Rhinox, Waspinator, Terrorsaur, and technically Inferno, as the Scavenger toy was initially intended to be his TM form. 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 McDonalds.
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.
Beast Wars cartoon continuity
Within the context of the television series, only Cheetor, Dinobot II, and Blackarachnia are 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, although it was apparently found by Megatron at one of the Vok sites on Earth and perhaps altered to suit his own purposes.
Optimus Primal and Megatron also received new bodies in season three, though only Megatron's toy was labelled a Transmetal 2. (Optimal Optimus was a normal Transmetal.) It is unclear whether, within the story of the cartoon, these bodies are meant to be similar to the bodies granted by the TM Driver. Both of their new bodies were the result of merging -- temporarily -- their sparks with the sparks of their Generation 1-era counterparts, Optimus Prime and G1 Megatron.
Tigerhawk's toy is also labelled as Transmetal 2. His origin involves both the fusing of two sparks -- those of Tigatron and Airrazor -- and Vok technology. The two sparks in question were both Beast Era sparks. However, it is unclear whether sparks from different eras are fundamentally different, and whether this would make any difference when creating Transmetals 2.
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 the results of Megatron's cloning experiments. This roughly aligns with the cartoon story, as the first Transmetal 2 that Megatron attempted to create was his resurrected Dinobot, who was preceded by a number of dinosaur-mode clones of that fallen warrior. This cloning experiment angle also helps to explain the presence of Optimus Minor among the many TM2 toys who were completely new characters.
Unlike Transmetals, most Transmetals 2 did not have a third mode. Only the largest TM2 toys -- Tigerhawk and Megatron -- had more than two modes, but these extra modes were not especially well-implemented, and were never featured in the TV show.
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 travel back to the Beast Wars era (early in Season 3 of the show, around the episode "Changing of the Guard") and activate the remainder of the stasis pods ejected from the Axalon. The Transformers who emerge from these pods include normal beast-form Transformers, Transmetals, and Transmetals 2.
Basically 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. One possibile explanation, not supported in any way by the canon, is that varied levels of shielding in the stasis pods allowed different protoforms to mutate to different degrees when exposed to the surge. However, it should be noted that among the cast of the TV series, it was the Transformers who had come from stasis pods in the first place that appeared to be immune to the effects of the surge. On the other hand, the only protoform removed from a stasis pod after the surge that we saw in the cartoon was Transmutate, who may have been Transmetal. (Rampage was also a Transmetal, but he was not stored in his pod as a protoform. Presumably he mutated by exactly the same process as the Earthbound Transmetals.)
