The Craniacs are a series of robot drawings that are obsessed with collecting futuristic devices, and dwell in Futuredome, a place where terrible future ideas go. Whenever a new Craniac appears, the last version is deemed "obsolete," then shuts it down and puts it into a freezing tube, to keep it into a frozen state. The versions are Craniac 1, Craniac 2, Craniac 3, and Craniac 4 (who was drawn by Rudy to shut down Craniac 3). Craniacs 1, 2 and 3 are now shunt down & put in freezing tubes. Craniac 4 became a ally with Skrawl in the episode "Double Trouble."
The 1st Crainiac was created by some sort of inventor and the 2nd one was created by another inventor and the 3rd one was the last one.
Crainiac 3 discovered the magic chalk and threatened to torture Rudy if he did not tell them how to use the chalk. But, Rudy shut down 3 and created 4 and 4 has been trying to get rid of Rudy ever since.