Finally here comes my report on
Bayne the purple Demon Dog.
The purple demon dog can be found in Karazhan, but he is level 71, the other purple demon dog is Bayne, a level 8 rare in northern Tirisfal.
Demon dogs became tameable with Cataclysm, belonging to the family of dogs. Their ability is "lock jaw": they hold their target in place for four seconds (like the "pin" from crabs).
Nerdanel was ways too low for Karazhan, so I tamed Bayne. She has a white/pink/purple pet theme in her stable and I thought he would fit in quite right. I never had to search for Bayne (already tamed him a few times on other hunters) and Nerdanel found him on the second spot she was looking for him. He is really big, so no way you could miss him. Seems like nobody wants to tame him or does quests in Tirisfal and kills him.
Appearance:
Bayne is NOT purple for me. I think that theses guys

look way more purple than Bayne.
Bayne (I named my dog Bayne, too, the only name that fit for me) is more like a rose, light pink with a green mane. His colours change from the light around him. I really like his colour in Tirisfal when I tamed him, but in Orgrimmar or during my quests he always looked kind of pale, grey or like skin. That is not really my first choice as colour - I thought about replacing him with the "blue" dog, 'cause this one has a really purple mane.
But there are good things about this demon dog model in general. I really like their faces. Bayne just has such a variety of expressions, that's totally awesome. He can look menacing (during fights), aggresive (depends on the angle), proud (while standing still) or like a happy puppy (during his animation). This variety gives him a lot of personality and made him grew on me very fast.
What was already mentioned a lot (also in other threads) is, that the demon dogs really are small. Bayne shrank a lot after taming and they look tiny next to almost every hunter (apart from goblins and dwarfes). I had one with my Tauren and abandoned him because he was almost smaller than her hoof. Please Blizzard, make these dogs a little bit taller, they look like a mini pet!
Animations and sounds:
Bayne is a quiet pet. His sounds are a very soft snarling or growling, I never really noticed them until I paid more attention. So nothing annoying, no barking or howling - but nothing special, too.
Bayne's idle animation is to make a little leap, like he tries to catch something in the air or wants to have a puppy cookie. When he's doing this he looks really cute and his expression is one of a happy little dog (picture top row on the right).
His run is a little bit strange. He galopps like a dog would, but makes himself really flat when he stretches himself out and his paws just move kind of odd, very far under his belly.
While fighting he leaps at his target (almost like his idle animation), bites it and hits with his right paw.
Swimming animation: sadly none. *sigh* He just stands in the water like on the ground and does his idle animation. When Nerdanel was swimming he was trotting at her side, but REALLY fast. No galopp this time, but trot in such a speed, just ridiculous. While birds have their animations slowed down to grace it's upside down with Bayne.
Fighting:
Bayne is small, so I lost track of him as soon as battles got a little crowded, I marked him with a dart in the first picture (above) to show how tiny he is. I turned off his lock jaw ability in dungeons, because he has to cast it and looses dps because of that. I did some pvp with him, too, where I put it back on autocast and we were able to catch the flag twice (was in Warsong), he held my enemies away from me and I ran away with the flag

I think he was quite annoying for alliance in the battleground, his size helps him not to become an easy target, I think mostly they didn't even notice him.
He didn't hold aggro really well, but I mentioned it before, no pets hold Nerdanels aggro when she doesn't send them in first. They just don't have the abilities to help them yet. I never died, so it's fine the way it is.
Conclusion:
Demon dogs are really nice companions. I enjoy having Bayne at Nerdanel's side. I never see the "non runed" ones on my server, but they really deserve some love, too. I will perhaps change Bayne into a blue one, just because I think one of them would fit better in Nerda's stables. Their ability makes them good pvp pets, but depending on how you specc them, they can do good dps, too. I like their animations and expressions, I personally prefer them to the rather stiff mastiffs. They just seem so full of energy and life.
I'd like to ask for my last roll. Next week my flight goes to Sweden where I'll spend one academic year and I don't know how much time I have for wow there.
Name: Nerdanel (Belf)
Level: 44, but pets till level 46 are okay.
exotics: not yet
restrictions: can I perhaps get something that fits in my white/purple/pink stable theme? it's not necessary, but I would appreciate.
rares I have: Snort (hyena), Arash-ethis (wind serpent), Mazzranach (plain strider), Ashtail (fox), Nix (wasp)
hard to get pets: white ghost cat, grey moth from Azurmyst
edited for some optical changes