I must have watched this somewhere between 2005 and 2010, it is a feature-length film and I think it was an anime. I remember one scene from it where a person looks through a glass at a spider-like character climbing on said glass, at which point the spider looks at the person behind the glass with the eye in their mouth. As far as I remember this spider-like character was out to get the one behind the glass. It looked mostly human except for the spider legs growing from its back (dr octopus style) and the “eye in mouth” thing.
I remember this anime being about mechas but I might be conflating two different things. In general, my memory of this is very fuzzy, so some things from my description might not be accurate.