The BluRay calls him an elder primal, but that's a typo. His name was Midgardsormr and he was a king among dragonkind.

He's called the Keeper of the Lake because Eorzean legends state that Althyk and Nymeia, eldest of the Twelve, created Midgardsormr to guard Silvertear falls. When the Empire attempted to take Mor Dhona in 1562, and thereby control the great conflux of aether at the heart of the realm in hopes of preventing anyone from using it to summon primals (which they'd fought on another continent and had no desire to see ever again), it was like the gods themselves were validated when Midgardsormr appeared (they say he rose from slumber beneath the lake, but we never actually saw it, so, who knows).

Countless dragons from Dravania hailed the lord of the lake's call, and they united against the invading Imperial armada. Midgardsormr coiled himself around the Garlean flagship Agrius and dragged it to ground, but the crash ignited the ceruleum fuel tanks on board and the resulting explosion destroyed them both. The wreck you see in Mor Dhona, made of the bones of Midgardsormr and Agrius' wreckage, is what people currently refer to as The Keeper of the Lake.