Mugen is a redeco of the ArmadaAir Military Mini-Con Team Mini-Con Terradive, transforming into an aircraft similar to the Cobra Night Raven S3P recon stealthjet (which is based off the Lockheed SR-71 spyplane). He also has a third three-bladed-claw mode. A 5mm peg was added to each of his heels so that Mugen could be used as a hand-held weapon by most larger Unicron Trilogy toys as well as be mounted on a Powerlinx plug. (Ironically, the Cybertron incarnation of Wing Saber is unable to hold 5mm-peg accessores in his hands.)
He and the other Caliber Mini-Cons were only available in July of 2005 from select stores in Japan as special promotional giveaways. Mugen could be obtained with the purchase of any Transformers item only from "Joshin Denki" stores, a move that made Mugen more difficult for fans to acquire than the other two, as the chain had no stores in the Tokyo area.
According to Doug Dlin, "I initially thought [Mugen] was the word 'infinite,' but Ichikawa-san pointed out it can also mean 'illusion' or 'misty illusion' (thus implying ninja-like deception and stealth) if different kanji are used. It's difficult to confirm which of these three was intended without official source confirmation, though."