I don't think there's fixed spawning times now, well at least not for this pet.
Used to be easier to get one on server resets, but this has been hotfixed in 5.1.
And yeah, he's pretty good of a pet, especially considering he's the only Undead with a Humanoid attack, making him particularly good against Dragonkins with Dragonkin attacks (most dragonkins).
I believe this thread should have been posted in Pet Collecting though
Plenty of threads about hunting this guy down in there anyway, you might want to check it out, maybe they have useful tips for you!