So far there's only one noticeable pattern: rare spawn points.
Keep in mind that this doesn't mean that the Unborn Val'kyr is directly linked or affected by any rare spawns, but they may share the same or similar spawn locations.
Or it could all be a major coincidence. We have such little information on where the pet could pop up, so it may just be natural that players tend to wander towards already known "hot spots" (aka: rare spawn points) where the val'kyr could just happen to be. Other non-rare spawn ares are highly likely.
That being said, 3 out of 4 confirmed spawn locations for the Unborn Val'kyr have been uncannily close to rares spawns.
Icecrown - High Thane Jorfus' spawn areaThe last point was confirmed in Storm Peaks, "north of K3", but I couldn't determine the exact spot since a map image wasn't provided. As far as I can tell, only the two rare dragons spawn/pat near K3. If the player was even further north than I guessed, it gets closer to another rare spawn point. It's hard to say, though.
Scholazar - near Loque'nahak's spawn
Zul'Drak - close to Gondria 's spawn
Until we can collect more varied confirmed locations, there isn't much to disprove the whole "rare spawn" idea. More information might be impossible at this stage, though, since the PTR appears to be coming to a close "soon". A release candidate was recently patched in, so in the end we might have to wait for 5.3 to hit live servers to test our luck then.
I have no doubt that we'll eventually figure out the puzzle that is the Unborn Val'kyr. It might take longer than any other pet discovery though!