To me, Penguin is the main bad guy of the old Batman TV show.
Side Note: I have been struggling with how to refer to the show. It has been known colloquially as “Batman ‘66” for a few years now and I think that can be almost 100% attributed to Chris Sims. But that’s not the actual name of the show, so I don’t want to call it that. But I also can’t just call it “Batman” because that encompasses lots of things. So I guess I’m taking the less concise route and calling it “the old Batman TV show”. Thankfully I don’t feel the need to add in “with Adam West” every time.