The Pega.SYS is a legendary cosmic McGuffin in the shape of a horse which is an emanation of the Source Code, possessing the power to ride into "higher" realities.
In The Bellerophon Gambit, Acton Lord and Action Lord sought the Pega.SYS to enter the Real World (later revealed as Real World Minus One) to resolve the universe-threatening continuity problems introduced by retcons to Acton Lord's origin. They were opposed by the Fan.dom of the Alt.ra and his henchman Damn Yankee who sought the Pega.SYS for themselves.
With a specially built harness, Acton Lord captured the Pega.SYS in alt.greener.pastures and made his way to Real World Minus One, where he found the former Dial D for Dvandom. After getting him to fix the origins of all the characters originally created by the Dvandom Dial, Acton Lord stole the Dial for himself and, using its powers to augment the Pega.SYS, made a dash for the Scource Code. But he was thrown from the Pega.SYS when the Dial melted to slag, and the Pega.SYS flew upward back into the Source Code.