The "list" of mercs can be found under the folder RTW\Data\world\maps\campaign\imperial_campaign. There's a text file named "descr_mercenaries.txt". It regulates which provinces can produce what mercenaries, their "regeneration rates" (ie. how fast new ones pop up), recruitement costs, and such interesting little details as how many exp chevrons they start out with etc. Mercenary Hoplites recruited in the province of Sparta have a whopping 3 chevrons to begin with, for example.

If you make up your own merc units (like I did by copy-pasting the Amazons appropriately), or for example want to get the Cheat Oliphants around, this is the file to tweak to get them to actually appear in the game too.

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...you know, I just started wondering how crazy things would get if you made the Oliphants appear in, say, Mesopotamia and then played the Romans or someone similar... could be pretty interesting when you go East...