Monster Soul Vol 2

A group of monsters known as Black Airs come across of pair humans who are looking to make some money hunting monsters with bounties on their heads. The Black Airs make quick work of the bounty hunters who fail to learn their lesson as they become overly ambitious and attempt to hunt the Black Airs. This proves to be a terrible mistake and we get to see just what makes the Black Airs so special: enormous power and even stronger friendship.

Source: Anime News Network Encyclopedia