Actually, thats not true,
Guillotine, its just a little more difficult, as it is a bit different. First, you'll need MyWarcraftStudio(
Link ). This application has the ability to open and extract from the .MPQ files in your ../World of Warcraft/Data folder(.MPQ files are basically Blizzard's own proprietary .ZIP files). While there are other programs that can do this as well, MyWarcraftStudio is by far my personal favorite(especially for this application) as it has a built in preview window. Install this program.
If most or all of this thus far makes little or no sense to you, then I suggest you stop here, and stick with the pre-made model variants.
Now, once you have the mod installed, we can start changing models. I want to stress first that none of this violates the ToS in any way, as all of it is 100% clientside. Any changes you make will only be seen by you, and only while playing on that computer. Now for the fun part...
Take what you want to replace(ie. your pet white wolf). Open MyWarcraftStudio(MWS), open the Models.mpq(in your World of Warcraft\data folder). Next, open another window of MWS(doubleclick the shortcut/exe again), and open Textures.mpq(also in your World of Warcraft\data folder. Also, make sure you opened it in the SECOND MWS window, so you now have 2 windows, one of the model files, the other of the texture files). Now, go through the models and find the model type that you want replace your wolf with(ie. a raptor). Keep in mind that EVER instance of the wolf model(ie. every wolf in the game, and anything else that uses its model, like the warg) will be replaced by the new model you are installing. Now, find the wolf in the model window, note the name that the .m2 file is called, then go to the raptor, click on it to preview the model, then right click on the .m2 file and click export. Save it as the name of the wolf's .m2 file (Should by default save to the Data folder, where you want them).
Ok, so your wolf now looks like a raptor, but still has the texture of the wolf. Looks kinda weird, don't it? Ok, go to that texture window. Now, find the wolf section, note the name of the .blp file of the color of wolf you have. You may have to check a few and look closely, as these are the flat texture applied to the wolf mesh, so they will look really different. Just look for coloring patterns. Next, find the raptor section, find the texture thats the color you want, again, click it to preview it, then right click it and hit export, then name it the same as the wolf .blp file (Also should default to your Data folder, where you want them). Now, you should have a brand new raptor. It will still SOUND like the wolf, though, as if you want to make it sound like a raptor, go into the Sound.mpq file, and follow the same procedures as for the model and textures.
Also, a note: You CANNOT(and I mean PERIOD) replace a model that only has one texture (ie. a wolf) with one that has 2 or more textures (for example, most flying creatures have 2, one for the body, and one for the wings. Onyxia, on the other hand, has 3 texture.). The original number of texture must mach the new number of texture, otherwise the game will take an error(if you are going from 2 texture to 1. ie. Replacing gryphons with wolves.) or will simply not apply any but the original number of textures(if you are going from 1 texture to 2. ie. Replacing your wolf with a gryphon. The gryphon's wings will jsut be plain white, no textures at all). For example, you could replace your wolf with a panther, or a raptor, or a troll even, but not a hippogryph. You could also replace gryphons with hippogryphs, or bats, or chimera's, but not with Onyxia, dragon whelps, or trolls(god I want to ride a troll to and from IF. Ultimate sign of servitude, but 'tis not to be). You can check the number of texture in the Texture .mpq. Just look for 2 texture named the exact same thing except it will have _01 or _02 on the end, and every color will have its own _02(so don't confuse it with the different colors, which can also be just numbered. The 2nd texture will be SIGNIFICANTLY different than the first(ie a gryphon's body and its wings)) Only known exception is Ragnaros, since his hammer is qualified as a secondary texture, not a double texture, so he counts a a single texture model, and his hammer will always be the right color.
In order to change your character in his base form into something different, you will end up changing EVERY other member of that race(and any other race that uses that model) into the new mesh and, if they use the same texture as your character(unlikely, considering the possible variations) the new texture. If not, just like the wolf-to-raptor above, they will have the old texture mapped on the new mesh. To change you character, though, just replace the race .m2 file, and the texture file for your particular skin color(which may take some trial and error to find the right one).
It should be noted that these methods can also be used to change your armor and weapon(s) to look like anything you want them to. Just explore through the file trees if your curious what you can replace. Try it out, and if it doesn't work, all you have to do is delete whatever file you extracted to to make it not work out of your data folder.