There's global models (referred to as "races") and zone specific models.
Global models = Can be loaded in any zone.
Zone specific models = Loaded via that zone's _chr.txt file.
You can add more models to the resources/GlobalLoad.txt file but this will make players use your custom GlobalLoad.txt file. Otherwise they see the human male model as default when the model is not available.
Alternatively, I guess if you're making people download custom files to play on your server and don't want to use the resources/GlobalLoad.txt file, you could let them download a custom _chr.txt file specific only for that zone.
At any rate here's a link to the end all be all of where the models load from and what the file's name is to load:
http://www.jondjackson.net/EQ/Emu/EQRI.htm
Download and just open the .htm pages he has in the zip, it has all the info.
You edit the race in game with command #npcedit race (number of race) with the mob targetted.
Type #npcedit help for all the different things you can change on the mob.
hth