Irregular X (イレギュラーＸIregyurā Ten) is a group comprised of special Killer Machine developed as ultimate weapon of destruction.
Z/X Encyclopedia Entry
At one time, Administrator Denebola researched the technologies of the other worlds, and developed the ultimate weapon of destruction together with Administrator Arcturus. Because the weapon were deviated from the Killer Machine's principle of mass-production, they are given the name Irregular. Erase was converted with the Resource pattern left by Cait Sith who frequently infiltrates the Blue World, and the shell structure of Plasect that was obtained through shady means.
The project is frozen when Denebola joined the revolutionary army, and the 1st machine, Erase was hidden together with the design data for the 2nd machine, CyberRex and the 3rd machine, BrighTron. However, the development has proceeded in the location unknown to her by the god Tiamat who exposed and transferred it to Arcturus.