Let your jimmies be unrustled, friends. Anyone in this thread who says "They saw someone months ago with a mount" is full of shit. 100+ pages on the official forums for reference, about 30ish pages here.
They didn't drop , period. No, the hipster who linked "/script DEFAULT_CHAT_FRAME:AddMessage("\124cffa335ee\124Hitem:87771:0:0:0:0:0:0:0:0\124h[Reins of the Heavenly Onyx Cloud Serpent]\124h\124r");" to himself, and says in raid Chat "Whoa! I got [Reins of the Heavenly Onyx Cloud Serpent] ! But I don't have serpent riding!" don't count, and you fell for a sad troll. That's along the lines of "If his name is Invincible, why can we still see him?", and the ever horrid "Why do they call it an Xbox 360?".
Son of Galleon was at least a little easier to troll the masses with the Oddly Shaped Horn, which looked just like the Galleon mount but without the armor. No one can be faulted for that, as someone actually presented "something" to look at, as opposed to the "Just trust me" Heavenly Onyx Cloud Serpent crowd.