Actually, Nath, the undead in Parlainth and Vivien(sp?) would be quite like Shedim. And the ones in what would now be Egypt, setting up shop in the tombs would be similar as well. AFAIK, they didn't have direct correlation to the Horrors, but as the mana cycle rose, they started showing up.
From the way Caimbeul and Icewing were talking when he went to deliver his declaration of war, the way is shut. Perhaps the Shedim's home metaplane is simply too far from ours to allow them to cross over at this point, just as the Horrors are too far. If we posit that the Invae's home plane is closest to ours (though still alien enough that they require a host to maintain their presence here at this time), and the Shedim are further away, and the Horrors further still, then it makes sense that, as the mana level rises, connecting to those planes will become easier.
So, we can gauge the Horrors by watching numbers of Shedim?
Something to consider - have Dragons actually been off-world?
Off planet? No. Neither they nor the Immortal Elves are capable of leaving the Earth's electromagnetic field, as I understand it. Off of the Earth's plane, on the other hand? Certainly.
Not quite true. The Immortal Elves can go anywhere they please, but like all mages, their powers will suffer in the mana void outside the Gaiasphere. With Dragons, it is another story, as they are dual-natured, and the mana void will be a Bad Thing for them, unless they have sufficient astral armor (or hardened astral armor) to withstand the void. As it stands, I believe Lofwyr and Hestaby could survive a trip into space, as they both have Hardened Mystic Armor 20, so a Void of -12 (deep space) wouldn't harm them at all.
And yes, that means that there's always a chance that the picture of dragon bones on Mars from the Big D's will is legit. Don't know about the pyramid, though.