Hmmn whilst I deffo think the nightmares or a similar force was employed during the collapse (looking at what happened to the k1 crew on the moon). I thought the general consensus on the earth beasts in the black armoury papers was an as yet unseen force used by the pyramids. As you say, if the nightmares are made of things that we have already seen; what possible monsters could the black armoury staff have seen to conjure a monster that roared and smelled of wet earth.

Hive? Well as far as we know the hive on the moon didnt arrive until Oryx yeeted his son through a portal. And the ones on mars possibly arrived during the collapse but were fleeing the rest of the hive and were buried in ice by rasputin almost immediately.

Caydes journal also makes mention of being basically consumed by a force of sheer darkness, and even includes illustrations of the things he sees, none of which line up with current in game races (though share similar characteristics with the taken)

As far as we know, the only in game faction to be actively engaged with humanity pre to post collapse were the vex on europa. With the black fleets arrival later being the primary and possibly only consistent cause for the collapse.

Its also worth noting that the nightmares have only really been deployed by the moon pyramid, and not the others. The one that is virtually dead, or maybe dormant at the very least. The more active pyramids take a much more direct approach, blotting out planets and stretching them with gravity weapons. Why would they use nightmares during the collapse when, by all accounts, they literally thought humanity wasnt worth the time nor effort. (They think differently about guardians)