In one of the earliest drafts of X2: X-Men United, the X-Men's terrifying robotic foes the Sentinels were part of the action. It was eventually determined they were just too expensive even to animate in CG, and by the time movie was finally finished, only one Sentinel head made a small cameo on the Danger Room.
But production designer Guy Hendrix Dyas and concept artists Ricardo Delgado both mocked up what the Sentinels might look like, and the results are quite interesting. Here's what almost terrorized Marvel's merry mutants!
First up, Dyas' work — note how his Sentinels transforms into a sphere for mobile mutant-mashing action:
And here's Delgado's version:
[Via Film Sketchr]