Back in 1979 it took a huge team of artists lead by H.R. Giger and Carlo Rambaldi to create the titular creature of "Alien". James at XRobots recreated it by himself using off the shelf materials and a lot of ingenuity. He also produced an incredibly detailed build log of the entire process.
This certainly wasn't a small project, but the basic techniques are something anyone can do. It's an inspirational example for anyone considering some Lovecraftian creature building.