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.