Gleipnir

Gleipnir (グレイプニル) is a manga series by Sun Takeda. It was later serialized into a TV anime series directed by Kazuhiro Yoneda.

Plot
Shuichi Kagaya is an ordinary student until he learns he can transform into a monstrous dog with a revolver and a zipper down his back. Unsure where his powers have come from and whom to trust, he risks it all to save a beautiful classmate named Claire. He learns he's not the only one with a secret; she wants to use his monster's powers—no matter how brutal the violence or bloody the action.

Links

 * Official Japanese Website
 * Official American Website
 * Wikia