The Tallgeese, or Prototype Leo, was the first combat-ready mobile suit and the ancestor of all modern mobile suits. It was developed at the Corsica research base under the auspices of OZ, a subsidiary of the Romefeller Foundation. The results exceeded all expectations, as the OZ-00MS Tallgeese proved so powerful that no ordinary pilot could handle it. The Tallgeese was essentially completed by AC 175, when its designers quit the project in reaction to the assassination of space colony leader Heero Yuy. OZ subsequently developed and began full-scale mobile suit production, using the Tallgeese as the basis for its smaller, less-powerful OZ-06MS Leo units.

This experimental mobile suit was designed for maximum power and performance. It was equipped with a pair of beam sabers and a shell-firing "dobergun," the Tallgeese's biggest advantage was its size and speed. However, being a large and heavy mobile suit, it required an enormous amount of thrust for jumping and maneuvering. Therefore, its designers equipped it with a tremendous vernier rocket system that gave it enough thrust to maintain flight on its own. However, the sheer stress and G-forces placed upon the pilot made the Tallgeese too much of a "wild horse" for anyone to handle, and the mobile suit was disassembled and put into storage, ostensibly to serve as a "museum piece."

Source: Gundamproject.com, New Mobile Report Gundam Wing