Again I really think it can be both creatures in a way, especially if we end up knowing it as the Leviathan. I think it's a safe bet it represents Jormundgand, or something from Norse mythology like Nidhogg at first, which Loki summons. It could still be used to refer to both characters if it uses traits from each. And if they make a habit of referring to it as Leviathan that would give them something else to call it instead of Fin Fang Foom or Jormungandr. I'm still thinking Loki or Thor will for sure at least say, or suggest it's the Midgard serpent. Or they'll reveal that the Midgard serpent was part of the Makluan species and FFF awakens because the other one's been awakened/defeated or whatever. Maybe Loki summons more than one monster/dragon. That post did say giant monsters... maybe Hulk gets his own to fight while Iron Man and Thor take on the more aerial based one.
All I'm saying is for movie purposes Midgard serpent might= Fin Fang Foom
with some slight redesigning.
This is how I could see it going:
I don't really think it would have a huge role in IM3 unless War Machine has to fight it in the finale. It could basically just stay hidden in a ten rings cave that seems to contain strange extraterrestrial ruins. It's revealed to still be alive, recuperating here, disguised among the ruins. Ten rings stumbles upon it and in a major twist the Leviathan/Serpent-Foom hybrid reveals it's from the intelligent Makluan species. They now have a common enemy in Tony Stark/Avengers so they begin the assault on Stark's company. Leviathan hands over a piece of alien technology stored in the ruins that he imbues with its own power(not the actual rings in this case), and entrusts it to the ten rings organization, remaining in the cave or doing lots of shapeshifting to hide throughout the film. Those dragons once took human forms in order to hide too, but I think that here the ability is passed to Ghost while Mandarin figures out how to adapt some of the other powers for the ten rings group before the finale. Leviathan could just stay in the cave, and is really using Mandarin/Ghost to get back at the Avengers through Tony's company. And if they merge the characters then they get the best of both worlds and never have to stray from the Norse mythology. They can just say it was the same dragons worshipped in ancient times in Asia.
I just think it'll have some shapeshifting/other magic abilities that will somehow get transferred to the tech Mandarin and Ghost use, allowing Ghost to rearrange his particles with Mandarin's technology to turn himself invisible or intangible, but not at the same time(a result of trying to make the battle suit invisible with the shape shifting magic). Ghost would go around invisible, and walking through walls and having Tony's blasts pass right through him. Stealing any tech they want. Causing all sorts of things. Hammer could be involved in some way, and then a powered up Mandarin emerges at the end, with or without Leviathan at his side.