Taxxons are big ugly cannibalistic centipede-things that serve the Yeerks. Visser Three figured out how to give them morphing powers, allowing them to turn into slightly different big ugly things.
One of the last Animorphs toys, the Taxxon transforms from an alien centipede-thing into a bipedal alien thing.
Although it was planned, this toy was never released in the United States. Because its planned release would have been near the end of the line, this led to a popular perception that it was never released at all, similar to the status of the Beast MachinesMegatron toy that eventually became Megatron Megabolt.