Sometimes that happens. It happened to me a long time ago. I dreamed about anarchy, even though I had no recall of hearing the word or knowing what it meant, but it fit the dream.

Either you didn't recall knowing about what copperhead snakes look like and your subconscious remembered, or it was one of those rare occurances when a dreaming person enters that realm that Carl Jung referred to as the "collective unconscious" and you picked it up from there.