Above are how the Sentinels looked in the “past” portion of X-Men: Days of Future Past. Previously, we’d only seen a quick glimpse of one in the danger room portion of X-Men: The Last Stand but as it turns out, the mutant hunters were meant to show up earlier in X2: X-Men United. Take a look at two artists’ renderings and imagine what could have been.
Here’s one version by production designer Guy Hendrix Dyas:
And another by concept artists Ricardo Delgado:
FilmSketchr has more and cites the “biggest reason the Sentinels were never used was because of the cost of computer animation at the time.” This is where that time travel stuff would have come in handy I guess.