G-O wrote:Also, I forgot that Zabuza also had shark-like teeth.
Shark features seemed to be either a Kirinin feature, or at the very least, a Legendary Seven Swordsmen feature. Suigetsu doesn't give any insight as to which it is, though...because he's a Legendary Seven Swordsmen fanboy: kinda' doesn't tell us anything.
G-O wrote:the question now is why did Zabuza leaves someone that strong behind?
From the way Suigetsu talked, he didn't imply that they even knew eachother, just that he knew of Zabuza and respected/studied/looked up to the Legendary Seven Swordsmen.