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Thrashers originally called Nefarious Machines were makeshift robots that attacked Ratchet in different ways. They had the ability to disguise themselves as a pile of junk, but when approached, would transform into its robot form and attack. They would hover toward Ratchet and attempt to slice him in half using their sharp blades. Advanced versions of thrashers had a small laser cannon connected to its back and would fire at Ratchet from a distance, only attacking with its blades if he came close. The robots in the Agorian Battleplex and on certain moons in the Corvus Sector had tethering devices to pull in Ratchet toward them for an easy swipe.
There was a glitch with these in the arena. If the Chimp-o-Matic was used on one while they were using the Tether, Ratchet would keep getting pulled towards them and not stop. This would cause him to go into the lava, die, and need to start the challenge again.
It is possible that they were made from scrap pieces of space pirates, seeing as their faces, body and color was similar to the pirates.