Inumata to Nekomata no Isshou

One day, a pet cat and dog named Buchi and Kotaro wake up to find that they have become yokai, supernatural beings blessed with eternal life. Now, not only do they get to spend more time with their beloved owner, they can stick around to befriend his children, and his children's children! But even as Buchi and Kotaro remain the same, their owner slowly begins to gray... (Source: Kodansha USA)