So much good advice, there is little to add... here are my few thoughts.

I like to look at classic lit as a good source. Shakespeare, Dante, Aristotle all new bad guys really well.

Don't forget the Bible. I used to hang with, well, "end timers" and we used to try to imagine what the Anti-Christ would be like. We decided it would be someone like Ghandi or Mother Theresa, rather than someone like Hitler or Stalin. The concept is an "adorable" beast. Someone who is loved by all would make a great beast.

Consider opening the story up. Could the villan be an older person? A woman? a child? perhaps a person with a disability? a person of color?

Bear something in mind when it comes to the visual aspect. "Bad guys wear black" has been beaten into the ground. Just like sunglasses and black leather. Even in a film like "Twister", the evil meteoroligists (I know, I know, I didn't write the movie.) had black suits, sunglasses, and black vans. Audiences expect to see certain things. That is something you can use or disabuse, but be mindful of it.

For the creature it self, I'd look to the old game Dungeons and Dragons. There was a monster called the Githzari that had a great look.

Once again I'd like to point out that what a character looks like can change drastically from the beginning of the writing to the end. For example, a script we are finishing up on started with an old man living with a retired friend in an apartment. Six months later, this old man has been rewritten into a twenty something stock broker living alone.

The story can lead you in many different directions as it is written. Dwelling on looks now only stalls the writing. Here's one question: Do you know how the story runs from beginning to end? Not necessarily in great detail but in a broad sense.

You need to know this before you make plans on what your characters look like. Unless you are the one deciding that, it may not be your choice anyway.