Earlier in patch 5.3's PTR cycle, developer Cory "Mumper" Stockton revealed the Unborn Val'kyr, a new wild pet coming to Northrend. That very night, players began scouring the PTR for any trace of it -- to no avail. A few days later, Stockton asked users on Twitter whether or not they'd found it yet. They hadn't, but as it turns out, at the time of those tweets, the pet had not yet been activated on the PTR at all. Whoops!
However, it has been active for quite some time now, and the community at large is still not entirely sure how it works. According to the information we've been given, the Unborn Val'kyr is not a pet that is randomly spawned, but rather only spawns under specific, unique conditions. The pet has now been found and caught, but the conditions under which it spawns to begin with are still unknown.
Since the pet was implemented, the pet battling community has been hard at work trying to unlock the Val'kyr's mystery. Complicating matters, the pet isn't restricted to any single zone in Northrend. It can spawn anywhere on the continent.
The pet's description reads:
Taking that description into consideration, as well as the nature of the Val'kyr themselves, players have tried killing Northrend rare spawns, killing themselves, killing other players, and a variety of other death-related activities. However, it was all for naught. Mumper later revealed that players are unable to fulfill the spawn requirements at all. The spawn conditions are realm-based.
Val'kyr are considered "unborn" until they vanquish a creature of high nobility.
Since then, players have been tracking all manner of things in Northrend -- the val'kyr raids on the Argent Tournament, the beginning and end of Wintergrasp, and the weather. So far, according to Wowhead's Perculia (among others), none of it appears related.
Until we discover the conditions for the val'kyr's spawning, the only tactic for finding and catching it is to sit and wait. It appears eventually. Why? Nobody knows. Not yet, anyway.
It's been a very long time since World of Warcraft has seen a mystery like this one, if it has ever really seen one at all. The hunt for the Unborn Val'kyr, at least for now, resembles the nostalgic days of early video games, when mystery was plentiful and nobody knew the truth from the schoolyard fiction.