Minsc had the incomparable Boo, after all, and though surely no critter can measure up to the example of the noble Miniature Giant Space Hamster, would Cailean have a familiar/companion animal/pet/whatever-they're-called-for-rangers? Like a dog to help keep watch for those bounty hunters? A hawk to scout? Or might he not want to endanger an animal by putting it in harm's way?
Though if you can take the recent thread on what you did with your CHARNAME's familiars as any indication, maybe many players would want something you could stuff into a backpack. Like Boo! (Sort of.)
(Between seeing a guy on the train last night that looked rather like Cailean - modern clothes and no pointy ears, mind you - and playing with the Kidnapping of Boo quest from UB, I've been thinking about this lately.
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Edit: Forgot to add that I'd be leaning towards something like a hawk/falcon/whatever, which'd be neat but a very good "backpack rider" as well - assuming such a thing is possible. Nice for scouting, then tuck it away for safety. Any other ideas, or am I overlooking anything?