Lion Turtle - The Deus Ex Machina of Avatar the Last Airbender, but it has been foreshadowed in The Library episode when Aang sees a scroll with an illustration of a Lion Turtle with a human.
Aang: Hey, look at these weird lion turtle things.
Sokka: Eh, I’ve seen weirder.
Aang: I learned there was another way to defeat him and restore balance. I took his bending away.
Toph: Wow, who taught you that?
Aang:A giant Lion Turtle.
Toph:You have the craziest adventures when you disappear.