This is my personal view on one possible explanation for the Raven.
Disclaimer - obviously the devs of ACM never intended this to be the case but I don't believe they ever even had an explanation for the Raven's origins and just wanted a cool new boss Xeno without putting any real thought into it.
The Raven[]
Raven concept art/model, possible undamaged base form of the caste.
A large, intelligent, powerful, durable, and aggressive Xenomorph, ostracized from the others deep in the sewers and capable of commanding lesser castes? This description reminds me a lot of another particular caste, the Praetorians who share similar traits and are ostracized from the hive when they begin to develop into Queens. The Raven is of course, not a Praetorian... But what if it was something similar? I believe that the Raven could be a damaged/battle-worn or otherwise experimented upon individual of a new caste fulfilling a similar role to the Praetorian... But not for a Queen.
The Lictor[]
I've nicknamed this potential Praetorian cousin caste the Lictor, another type of Roman bodyguard much like the Praetorian's namesake. So, what is a Lictor? The Lictor is the "male" equivalent of their Praetorian sisters, bridging the gap between the lesser castes and the monstrous King. While not really a true caste (though the Sideshow maquette's description does state that it is a "canon character"), having only debuted as an old Kenner toy and a Sideshow maquette/Prodos game-piece, I would love to see this magnificent looking caste canonized. I understand why some people don't like the idea of a King Xenomorph, but you have to understand that these are merely nicknames given to the castes by humans and don't necessarily reflect the true nature of the animal. I personally envision the King as a sort of "General" caste, a hulking, raging beast of destruction that leads the lesser castes into battle. Much like the Raven is seen doing in ACM and what the unidentified giant crested Xeno is seen doing in Armageddon.
We see in ACM that the Xenos are bringing out their more combat adapted castes, with Spitters, Crushers, Boilers, etc. Probably as a response to all the military and paramilitary operations on Acheron at the time, so I believe it would make sense that the Queen wants some backup, especially after she is captured. The Prodos Games description backs up the idea of it being an offensive caste. And so, this is where I believe the Raven fits into the XX121 hierarchy. What's your opinion?
Digibash Concept[]
While as can be seen above, I originally used one of the pieces of concept art/3D model as my basis for what a Lictor may look like. The only problem with this is that this concept art is quite different from the actual look of the Raven and so using new concept art of an undamaged Raven head that I found (pictured to the right), I have digibashed it together with the full-body concept of the finalized Raven design to create what I think a Lictor would actually look like. Any feedback would be appreciated.
ADDENDUM - As pointed out to me by Freighttrain, The, it perhaps would make more sense for the Lictor (in his headcanon he has renamed the Raven as the 'Vanguard' caste instead, as well as renaming the Royal Guard - juvenile Praetorians - as Lictors) to come from a Lurker instead of a Warrior.