So what exactly are they? Are they like ancient powerful beast they sealed up?
Guardians are protectors of the vaults, the power of the vaults.
A lot more is explained in the Eridium texts
I was wondering that too. The only feasible answer I came up with is The Destroyers offspring or minions. They all seem to have a shapeshifting ability and evolve to match their environment. Power battery theory might work but why would it attack Eridians if it was made by them.