Who's This? The big robot Mr. Poseidon controlled in the Forgotten Villains was actually from an entirely different story/franchise/decade.
How you could have heard of him: Showcase Presents the Challengers of the Unknown is a cheap reprint trade, and you don't have to dig very far for Ultivac's issue. The robot also appeared in Young Justice #50 and of course, Resurrection Man #25.
Example story: Showcase #7 (March-April 1957) by Dave Wood and Jack Kirby, reprinted in Challengers of the Unknown #75 (1970)
Who else? I hope you liked Forgotten Villains Week. Up next on Who's This? a Golden Age villain. Those are as good as forgotten too, aren't they?