The entire first blood.

[Hinoto-no-Ryuji]

I'm actually curious at the possibility that the identities of the last two unknown first brood are being purposely withheld. None of the in-game lore make any mention nor reference beyond the first 5 dragons, and the story/characters act as if they don't exist...

Keep in mind, though, that we never got any direct references to Tiamat, Hraesvelgr, or Ratatoskr before Heavensward either, and none of the Dravanian dragons make any mention or reference to their Meracydian kin. It's not too hard to imagine that not all of the First Brood were as close as Bahamut and Tiamat, or Niddhogg and Hraesvelgr and Ratatoskr. Maybe they just drifted apart across the ages, as they sired their own broods in their own corners of the world.