Many of these models could be used for tameable beasts, companion pets, or just something that could benefit the WoW population as a whole. Such as tameable beasts or companions.

1. The Dragonfly.

There's been one topic on this in the past, back in 2010, but I figured it deserves to be mentioned here.
The "dragonfly", categorized under "silithidmodels", has absolutely zero animations but is intriguing in its own way. It looks like a worthy addition to the silithid family, and if it had animations I would definitely tame it. However, if Blizzard ever wants to add a tameable dragonfly family (that's a hint, Blizz!

2. The High-Definition Yeti.
Apparently there are HD versions of the classic-model yeti skins. Take a look at the difference.

Ugly, stinky, outdated yeti

Glorious, fresh, much more updated yeti
These yetis alone give me a great deal of hope. It's been obvious that when Blizzard adds new models for a creature, they don't replace the old skins--there's always someone who prefers the old ones. However, that's no reason to completely ignore the older skins! One user (I forget who) posted some excellent mock-up pictures of the old-world wolves with "smoothed-over" fur and non-jagged claws. This is basically what's been done to the yeti model here- touched up without changing the overall feel! I have no idea when this model was added, but if it was recent this could be the start of a model makeover.
3. The Peacock.

How cute!
These birds were added in either 5.0.5 or 5.1, sitting neglected in the files. There are at least four colors, all of which are equally attractive and striking. Hopefully they'll become companions in 5.3, but it makes me wonder why they weren't implemented earlier?
4. The Bubblecreature.

Positively bubbling with energy!
This is an interesting one. It only has a few animations, and is labeled as "bubblecreature". This could be a companion once more animations are added, as some sort of elemental pet.
5. The Undead Nerubian Beast.

What a strange creature! This would make a great Nerubian enemy once animations are added. One of its eyes is glazed over, and it's covered in bandages and spikes. That screams "undead" like nothing else.
6. The Nightmarish Horror.

Wait... what? What is this thing? It's kinda cute. But something about it seems... off.

It's looking at me!

No, no! Get it away!

Į̳̗̦̳̮ͭ̾T̫ͯ̐̃̓̐Ō̮̻̭͍̝̭ͭ̾ͥ͋̋̈ͫS̰̖̯̯͈̔ͮͨ̽͐͘ ̯̟͙ͨ͊́͞S̝̭̠̞͕̙̆͐̚T̸̹̥͙̈ͪ͒̃ͥ̄ͯA̙̻̱̩͉̮͆̒̄̑ͅͅR̸͔̝̺̲͕̘͂ͪ̃͢͠I̸͚͇̜̮ͧ̀N̵͚̯̹̘̝̿͋Ḡ̶̛̹̫͔̭͝ͅ ̵̦̺͕̤̥̟͕̺͐͡Ş̛̙̿̿͆̇̑̎
̷̵̬̩̬̫̺̼͌̂̎̈
̮̲̱̟̥̖͖̣̖ͤ̉ͣ́ ͚̖̘̭ͯ ̛̥̮̞̯̮̭̯̃̓͒͜ ̢͕͇̥̘̫̰̅̐̆̐̆̃̏M̢̱̭͚̤̥̠̞̤͆̔͌̚͡͡Y̸̢̟͚͎̟ͬ̅ͬͧ̒̋͋ͤI͓̤̠̳ͯ͛͌̌͒ͫ͐͜͞N̮̯ͥ͘Ṭ̵͙̥̬͖̹͌͊͋͝O̶̪͙̖̥̣̬̫ͬ̆͑͝ ̜̻̲̑̈́͑̾ͬ̅
Ơ̡̯
̴̤̹̺̫̠̝̼̣̱ͤ͂ͭ̂͂̄ͥ́͠ͅḨ̷̛̥̺̘̘̠̓ͭͥ̆ͪ̉̊͌E̶̴̷̡̧͉̭̣̫̯̜̾ͯ̈͐̿̉͛̿͐ͭ̈́̉L̵̡͎̫̮̥̙̼͖̭̭͍͔̫̹̃̉̓͐̄̓̒̌͛̐̆̽͆̂̀͟͞ͅP̨̈́ͤ̒͑ͭ̆ͩ̅͛ͫͨ̒̋̓̀͟͢͏̹̪͕̦̤̘̳͖͚̣͕̭͎̰̫̤ͅ
̩͖̪̭̖̜̭̯̟̟̣̟̤̱̗̺͇̀̀̉̊ͥ̅ͤ̄͛̽ͧ̆͗ͧ͛̉ͥ̐́̚͠Ï̇ͧ̉
My Modelviewer crashed at this point. Don't believe me on the demonic capabilities of this monstrosity? Look it up for yourself. Look how it floats, possessed, twitching at odd angles. It's called "raccoon.m2" - The regular raccoon model is called "raccoon2.m2". Now I can see why they made a second one.
Did I miss any other ones? Care to share thoughts about these mysterious missing models? Post in the comments below!