it was a sephiroth clone with his memories, they were all clones of Sephiroth who supposedly somehow survived the first reunion which might account for why they are so strong

or another theory could be that they're all just spirits and each one symbolizes a different aspect of Sephiroth and that they materialized when Shinra tried to take the head from the crater.

the Bahamut he summoned was Bahamut Sin which was not in the video game, you can check out it's toy if you don't believe me