Bahamut and Eikons and Ascian[s], Oh my [hint: this has spoilers and speculation]

[Platinumstorm]

Keeping you in the dark over "major events" is a plot device used by many MANY (see: almost every) games, movies, books, television shows...
Not everything is revealed at the start.
It's a shame that we have to debate on the identity of an unknown dragon in the opening cut scene?
Yes there are parts of the game that were annoying, to say the least, however, choosing to leave the identity of the dragon a mystery is not one of them. Especially if knowing (or in this case not knowing), and subsequently finding out becomes a major part of the story

The events of the cinematichappened 15 years ago and had widespread consequences including transforming the world and interrupting the power of the Garleans. This is not a mysterious plot device save for their expulsion of the eikons. The dragon is incredibly large, it would have been well known to the people in Eorzea, and even if it hadn't, the people would have given it a name after such a historic moment that transformed the world. The cinematic is backhistory designed to give players an idea of what has happened in the game. The mystery comes afterwards.