Thanks for the nice comments, guys. The Brainiac is completely build from scratch.
Here's the Recipe:
Arms: GW Plastic Chaosspawn
Face: GW Plastic Ogre Head
Legs: Star Wars CMG: Huge Crab Droid
Gun: AT-43 Fire Toad Minigun
Brain: Wind-up Walking Brain Toy
It was supposed to be a "Spider Mastermind Demon" for Doom the Boardgame and stood unpainted on the shelf for years.
This is how the original Mastermind looks in the old pc game:
This is the wind-up toy used for the brain: