Click over to X3 Movie to see a maquette of a Sentinel that was prepped for X2: X-Men United.
Guy Dyas and James Jones designed the mutant-hunting robot to use as a model for the CGI animators. But adding Sentinels to the sequel would've cost $8 million, the designers told the site. Thus, it was cut from the film.
A neat note: the sculpture is hollow to indicate how the film's robots could retract themselves into disks. Director Bryan Singer previously told Total Film magazine: "The Sentinels are something we looked at for X2. I originally brought Guy Dyas, my production designer, on board to do Sentinel designs for me and he came up with a clever design and built a maquette of it, which actually sits in my home! But we felt, for X2, the concept of giant robots is a difficult sell. But it's definitely something we may have room for in the future."
In related news, although Colossus star Daniel Cudmore inked a deal for the third film, 20th Century Fox has yet to call him about it.