Technically, it is an alien. Scar was facehugged earlier in the film, but for one reason or another Anderson decided to have the chestburster be spawned after Scar had died (which is normally impossible).
What you saw was what people refer to as a "predalien." However, I fail to understand why individuals insist on calling it that, as xenomorphs have absolutely no uniform appearance. Just because we have seen xenomorphs spawned from humans hosts more than yautja does not mean that a xenomorph spawned from a yautja is something "different." A xenomorph is a xenomorph, regardless of its host.
But I ramble. That creature was a xenomorph spawned from a yautja. It inherited certain traits from its host (Scar), which is why it possessed the yautja's mandibles. It is still a xenomorph.
Yautja most likely reproduce in a manner very similiar to humans. They obviously possess genitalia, and their reproduction is definitely sexual, not asexual. We still do not know if they give live birth, nor what female yautja look like, but I have very little doubt that they exist.
And thus I clothe my naked villainy
With old odd ends stolen forth from holy writ
And seem a saint when most I play the devil.