Abara

Abara is set in a dystopian world, filled with large structures. The story revolves around creatures known as Gaunas, those who can shape bone like armor and weaponry around themselves by transforming. The transformation begins around the spine and then shapes layered armor. The story focuses on one such Gauna known as Kudou Denji, though he uses Itou Denji as a fake name earlier. (Source: Wikipedia) Included one-shot: Volume 2: Digimortal (two parts)