Hi,
If you want the Horde bike you need to have logged in Before September 30, 2014. Since this mount will be account wide there's a chance that you can flag your battle.net account by creating a free starter account and making a horde character on it in order to log in to the game.
Obviously I can't say for sure since there seems to be no way to tell if your account is flagged but personally I'd rather take the risk instead of buying game time on my main account for one mount that I don't really care about.
Hope this information helps someone!
(Get to Your Choppa! - World of Warcraft)