This article describes various allusions to Demons in the Smallville series.

Chloe: You can't keep running away, Clark. We all do things we regret. Sometimes you have to stop and face your demons.
Season Three: Phoenix

Dr. Foster: Lex, if you don't let your demons out, they could consume you.
Season Three: Perry

Lionel: Lex, even if I were the demonic father you've made me out to be, you know I would never do anything so sloppy.
Season Three: Crisis

Oliver: Nice to play. I just started asking myself who knew about Lex, about Toyman. The things I'd given up, what i tried to do. Then it all occurred to me. Dinah shattered the glass in the warehouse, Bart rescued me from the car, Victor faked the computers, and Watchtower kept an eye on the whole thing. Right?
Chloe: You were living like you had a death wish. You had to face your demons if you were ever gonna make it out alive, and I had to push you over the ledge in order to pull you back.
Oliver: Did you have to push with a 3-ton truck?
Chloe: I didn't think a tricycle would be a strong enough point.
Season Nine: Roulette

There seems to be a few demonic entities in the Smallville multiverse: