Trivia: The smile that Damien gives at the very end wasn't scripted. Originally, Damien was supposed to "look really mean," but Harvey Stephens couldn't keep a straight face: he started to smile, then laugh. Richard Donner decided that the smile made Damien look even more evil, so it stayed in the movie.
Trivia: Richard Donner was originally wary of sending the script to Gregory Peck. Combined with the original ending, in which Robert kills Damien, and the fact that Peck's son had recently committed suicide, Donner thought offering the film to him was distasteful. But to Donner's surprise, Peck agreed relatively quickly to do the film.
Chosen answer: The idea wasn't entirely Hollywood's. The antichrist is sometimes said to be "born of a jackal. " Jackals are mentioned in the Bible, and some interpretations consider them a symbol of something that is "not of God." In the movie, it means that the kid was scary as hell, literally.
Hobbes