The Warcraft twitter account just linked to a new page on the official World of Warcraft site regarding Invincible. This stallion served Arthas in life as his mount. Unfortunately, Arthas worked his mount a little too hard and slipped on ice. Arthas had no choice but to give Invincible a merciful death. (You can find more details about the story if you've read Arthas: Rise of the Lich King by Christie Golden.) When Arthas became the Lich King, he visited Invincible's grave and raised him from the dead in order to serve him.
How do you get him?
Oh, it's fairly simple to get. This pony can't get hungry, tired, or feel pain. Great for a Lich King to use when traveling around, right? All you have to do is pry it from Arthas' cold, dead hands. Beat Arthas in Icecrown Citadel on heroic mode, and Invincible will serve you.
But we don't know if everyone in the raid is going to get one (like Ulduar drakes) or if only one mount will drop (akin to killing Sartharion with 3 drakes up or the Mimiron head mount from Yogg-Saron).