Clayface appears in the "Caped Crusader" episode 2, "... And Be A Villain.” He is played by British actor Basil Karlo, who has the talent but not the looks to be a leading man.
Karlo's name, horror movie resume, and features are references to the real actor Boris Karloff, the actor most famous for playing Frankenstein's Monster.
"Boris Karloff" was a stage name. He was tall with both a large head and big eyes, and his appearance remains the default look for Frankenstein's Monster almost a century later.