The idea of marking their foreheads or 'blooding' as it's referred to is from the Aliens vs. Predator novels/comics by Dark Horse. They were released in the early 90's, after Predator 2 came out. Shane Salermo (screenwriter) and Paul Anderson just kind of hijacked the idea and exploited it in the film version; in the novels, it's so much better explained and executed.
That 'lightning bolt' design used in the movie is the same look referenced in the novels/comics. Supposedly, each Yautja clan has their own emblem.