How Do I Find Hidden Units?

Recently, I read on come forum (the specifics escape me) that vanilla RTW had several units that were included, but not implemented. These included a Briton berserker that wields a giant sword and Indian elephants. I have enough experience to add these units to the game, but I have failed to locate them. Would anyone happen to know what directory the models, textures, animation, etc. are located in

First you need to unpack the Data\packs directory using Vercingetorix's xpak:
https://forums.totalwar.org/vb/showthread.php?t=40313

You'll need to explore the unpacked directories to find exactly what you want.