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So we have seen Leviathan in the trailers. Only one so far. So what's the deal with summons. Ion an interview last year in 2013, Nomura had said that summons will be in ranks. Leviathan being a top-ranking summon is why he's so large. Also that they have to be defeated before they can be acquired. Levianthan seems to be the Summon for the water city "Accordo".
tumblr_moriajVffT1rmlmsso1_400.jpgNow I have a theory that the nations have summons attached like check out Lucis crest.
It resembles odin/diablos/doomtrain. Possibly doomtrain more than anything. If so the last time that was seen was in FFVIII. So that would be interesting if that is their summon. This is possible as Nomura has already went back to his VIII design of behemoth. As we saw in the gameplay trailer, it could be a bit of stretch though because behemoth hasn't changed too terribly much.
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Then we have Nilfhiem's which looks like a dragon which could stand for bahamut. This would make sense as they are supposed to be the antagonist family/country. So having a bad ass king of dragons would fit this country best.
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Last we have the hooded man. He has a wolf on his shoulder (stark? ;P) Which could mean he's from another country entirely. Or it could mean he's from a lesser house and therefore his summon is smaller and ranked lower. It could possibly stand for Fenrir.
If that's right then that means each sigil for each house would have a summon. And if you have to beat the summon to acquire, maybe this is also a way for you to get other houses to join you? Maybe. Also Accordo would have been Leviathan and we see Noctis running to fight him. What could possibly have happened is that he got there when Nilfheim was getting attacked and they used their crystal to defend themselves?
What summons do you want to see? What do you think of this theory?