"Troll" refers to "trolling" a method of fishing where a lure is dragged behind a boat moving thru the water to entice the fish to bite. An Internet "troll" is someone who tosses out comments with the intention of getting people to "bite" - That is saying things you don't mean just to see who will argue with you. It does (and never has) referred to a monster that lives under a bridge. There is no context of it's usage that would support that.

@Brock - I think you are trolling, which refers solely to your actions, and not your (supposed) physical appearance.