Kingpin
The Kingpin is an alien enemy cut from Half-Life. While it lacked convincing AI and gameplay goals, it was scripted nevertheless. It can be found in the game files. Twice the height of a average person, it was to have a massive brain filling its entire body, with four eyestalks spaced evenly around it. An early version was to walk on three chitinous legs with a clicking sound; a later version traded the rear leg for a thick worm-like "tail". It was to be found on Xen, in some caves. Its psionic attacks and psionic defense would have made it immune to missle weapons, although this is unconfirmed.
Read more about this topic: Creatures In The Half-Life Series, Creatures From Xen