Ok, first you need to download MyWarcraftStudio from http://www.curse-gaming.com/mod.php?addid=657
Once that is done, use it to open the model.mpq in your World of Warcraft/data directory, find the model you want to replace and note it's filename and find the one you want to replace it with, extract it to the data directory. Now go into your data directory, find the newly extracted file and rename it to the one you want to replace, boom now you've replaced the model (no MPQ editing required, just extracting and renaming)
Now do the same with the texture in Texture.mpq, some textures and models are in Patch.mpq (anything added after launch)
Here is where it gets tricky, some models have multiple textures for them and you cannot replace them with simple models, for example if you try to make your voidwalker (a single skin file) into Onyxia (3 skins) you will have 2/3 of your Onyxia appearing as matte white nothingness, but most regular mobs work - including naga, and ancient core hounds.
http://img.photobucket.com/albums/v287/rugburn123/censoredbeast.jpg
If you want to do the sounds for a pet look around in Sounds.mpq under creatures and in Speech.mpq, like I did for my succubus and felhunter (I like the high elf model succubus for some difference from the norm, she has some neat attack animations)
You can also find some cool looking items, I replaced my warrior's armor and axe with these, and shield and 1haxe
http://img.photobucket.com/albums/v287/rugburn123/coolgear.jpg
I've never seen that axe before, seen the hammer on test once, but those are actually level 10 warrior items with different graphics
This could be really useful for creating wow movies (if you are into that), if you wanted ragnaros to play a part, or have people wearing near-unobtainable super epic gear, that's the best way