DBCs + DB tables:
CreatureModelData.dbc
CreatureDisplayInfo.dbc
CreatureDisplayInfoExtra.dbc (used only for character models with equipment, not for creatures)
+
world.creature_model_info (don't forget this one!)
world.creature_template
Tadaaaa... You have working NPC with displayID made of new model. Google these tables and DBC files for documentation and I'm pretty sure you won't even need any tutorial, this is quite simple task to do. But feel free to ask if you get stuck somewhere.