Crygil's been getting to the bottom of the sea -- sea turtle, that is (sorry about that one). Some players wondered if it's getting picked up in just Northrend pools or all over Northrend waters, but as we originally told you in our guide to getting one of the game's coolest new mounts, it is supposed to come only from fishing in pools in Northrend. People are reporting it appearing from elsewhere, but straight from Blizzard's blue text, that's where it's supposed to come from.
As for when you'll get it, that we don't know -- all we know so far about droprates is that it's "extremely rare," which is about as rare as items in Azeroth get. We've heard from quite a few people who've gotten it already, but for any given catch, the chance is really, really small that you'll pull up a mount along with your fish. Still, it's not impossible, and especially if you're leveling fishing (which you should be -- it's lots of fun nowadays), it's definitely something to keep your eyes open for.