Automated soldiers who wiped out their builders and still wage war against the robots of the rival Cravic, a conflict in which B'Elanna Torres became entangled.
The Pralor are a species of robots known as Automated Personnel Units. They were built by a humanoid people to fight their enemies, the Cravic, who had built similar machines of their own. When the builders made peace, both sets of robots turned on their creators and destroyed them so that they could continue the war they had been made for.
In the episode Prototype, Voyager found a damaged Pralor unit and B'Elanna Torres repaired it. The robots then abducted her and demanded that she build a prototype so they could replace their losses. She eventually agreed, but when she realised the truth about the war she destroyed her creation before Voyager could rescue her.