I really like the Raven. It's so cool how he has incredible strength. I think the Raven, along with the Crusher, Predalien and Praetorian should work together as a great team. Well, in my opinion, they would do very well against the Undertakers from the anime Deadman Wonderland.
i think it was a xenomorph being experimented on, they wanted to see what kind of damage or gunfire could injure a xenomorph and see how they regenerated. if they did so, could explain the scar on its face, and its body became immune to what was used to injure it (bullets) much how like a body gets sick with a disease and then creates anti-bodies to combat it next time it comes around. the body decided it needed more armor and to be bigger, poof, Raven breaks out, spends its time lurking in the darkness waiting for Weyland to come after it so it can exact bloody vengeance
that or its the result of a crossover between Aliens and Resident Evil and the Ustanak was face hugged and the resulting creature was the Raven, hahahah, couldn't resist, but being seriously, i think the raven simply evolved to survive whatever experiments where being done to it. and broke out after it got its new body Armour and strength