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I really, really like the Skaven from Warhammer fantasy. They're just one of the most wonderfully unique factions in any fantasy anywhere, and twisted in an (ironically) very human way: rabid rat fanatics anyone? They're even sort of sympathetic in an underdog (underrat?) sory of way. I don't know if rats should be mounts in the core game - maybe. If there's a well-done rat people mod or giant rat riding mod then I would add it to my games.
I absolutely love the Skaven, and if they'd been in the game on the release of WAR, I'd rolled Destruction in a heartbeat.
Of course, that'd strand me with all the utter and complete niblets. But being a fanatic rat with a plague censer.. OH YEAH!