Okay, so the hydras were all dragons once, but they experiences that correcpond to their sin. The dragons were greatly affected by the hydra, but the hydra wouldn't have existed if bad things hadn't happened to the Hydra when they were dragons. It's a 'Monsters aren't born, they're a product of the cruel world.' Kinda deal. 

I have ideas for Noire, Charon' and Beelzebub, so hopefully I can surprise you with some of that, but to be honest, the rest of the hydra need fleshing out. If anyone's an expert at surprises it's you, here are what everyone was before they were hydras! 

Charon/ Water dragon
Noire / Metal Dragon
Beelzebub/ Shadow dragon (Had a brotherly relationship with Edge)
December / Storm (grey) dragon
Blizzard/ Ice + Snow dragon (dah, the Snow dragons were at war w/ the other clans and were facing a famine)
 Forte/ Wind Dragon (Spec. Forces, I have some idea...)
Leo/ (I don't really have an idea with him...)