Rare spawn pets do not appear as secondaries, even to themselves. No amount of Tainted Rat battles will net you a secondary Minfernal and so forth.
If anyone has any bugs with pets that I have not covered, feel free to post.
It's not there. No one has found its spawn, nor in the beta, it's highly likely Blizzard hasn't placed it in game yet. His sister in AQ isn't there either yet.
EDIT: It's tentatively reported that this critter only appears during the Feast of Wintersveil.
Found in the Shatter Scare Vale, a short jog south of Whisperwind.
Reports say that farming the big Infernals may yield a littleun, I personally haven't had it happen in many hours of killing. There seems to be CRZ errors. When you FP over, you see bunches and your heart leaps -- returning shows nothing there. Probable phasing error.
If you're gunning for a rare, my advice is to wait until a server restart, and log in on an alt in a low population, non PVP server. Minfernals await in droves, though they DO NOT respawn. Blizzard's maintenance time usually ends anywhere from a half-hour to an hour earlier than they said unless big problems, so keep trying to log on.
If you bind Target Interact in your Keybindings, you may nab the Minfernal in the tree with proper positioning.
Found below the Argent Tourney Grounds.
Tricky and rare. Shares the same possible CRZ error as Minfernals, showing up when you FP over but not there. Personally have confirmed they do not share a spawn timer with the other rare mob in the area.
I've found handfuls on my characters that have not completed any Argent Dailies. The most I have ever found was on the character that had completed the Quel'Delar questline (which I hope had nothing to do with the enormous amount I found as it's a very expensive 'fix').
My advice is to either circle on a low pop server (as I've seen up to 5-6 people circling with me on WrA) or to try the same server restart log as the Minfernals seen above.
Found in the cellars below Karazhan -- look for roofless buildings with a square entrance in the ground leading below. You can track them above ground to see if they are there.
Spawn at 12:00 am sharp. Be the first one there or be square, cause they don't respawn until the next day. Someone beat you to it or they're just not spawning? Alt-hop to a different server and see if they're there.
Cont. below are the weather-dependent pets...