I will just add a few things that might help you, because if I had never seen DBCs before, I would have no idea what to do even after reading Gurluas' post.
First of all, CreatureDisplayInfo.dbc hold displayIDs of NPCs. If you type .npc info on NPC and get its displayID, ID of its row in that dbc will be the same. So if Sylvanas has displayID 12345, you need to copy row 12345 in that DBC to your new custom displayID you want to create (for instance 40000, look at the end of DBC to find out which ID is free and not used).
As you can see here:
http://www.wowdev.wiki/index.php ... ayInfo.dbcthe 2nd column in that DBC is link to some ID from CreatureModelData.dbc. Lets say our Sylvanas has model ID 1234 there. Open that CreatureModelData.dbc and go to row 1234. Again, look to the end of DBC which ID is empty and not used yet, for instance it will be 4000. Copy row 1234 to 4000.
All you will need to do after that is rewriting model ID for your displayID 40000 from lets say 1234 to 4000 and rewrite path to model in model ID to your, lets say C:/Pony/Creature/Sylvanas_HE/sylvanas_he.mdx. It must be the full path to model (it doesn't matter where you put that model in patch at all, but you need to write its path here correctly and with .mdx postfix, don't put any .m2 or something like that there).
Looking for DBCs in locale folder is just suicide and too long time taking. Use this tool, run it in root WoW folder. After a few seconds you will see that DBCs were extracted and that maps are being extracted. You can just kill that app when it comes to this, you don't need to extract maps. There will be created folder dbc including all blizzlike DBCs.
And just if you want to ask... no, we don't want to do it for you. For sure we can, but we want to make you try to do it by yourself, so you won't need us next time/you won't need so much help when you will want to do something another with DBCs. Btw, if you want to use that displayID for NPCs, you will also need to put new versions of your DBCs into data folder of your server and you will also have to add new row into DB table world.creature_model_info with the same ID as you made (our 40000). Google that table's structure and you will know what to put there.