With the assistance of the ever-wonderful Wain, he who discovers rares, I was able to get into Karazhan to check out those last few demon hound models. I think the results speak for themselves.
All
three four of the demon hound models in there are tamable. Two of them, the Phase Hound and the Shadowbeast, are slightly transparent. In addition to their transparency, Phase Hounds are also the only tamable white demon hounds in the game (that we've found so far). Hopefully we'll see some non-transparent versions in the future.
EDIT: Turns out I missed one. If he spawns, I'll be checking out Rokad the Ravager. Will update if I find the blighter!
UPDATE: And he is! Beast Lore screenshot included with the list below. He's not a boss alas

Was kinda hoping for another boss tag pet, He hits like a truck though. Took me down to a good 60% health before taming finished.

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