This alien is weird. It isn't very big (about the size of a headcrab) and really doesn't have much of a purpose. He would have been a monster that is under water. He has a shooting attack which suggests he could have shot some sort of projectile, but it could also have had a short range attack.

archer.jpg (4732 bytes) An old shot showing some Archers attacking

archer0.jpg (58073 bytes) An old shot of an Archer in its idle sequence