Gokutora no Honey Bunny

Ao, a caring rabbit, one day meets a beautiful boy of the name Asuka who fled to Ao's town. Asuka is a tiger and the heir of a yakuza family from a neighboring town, and Ao and Asuka's family become friends. They are separated when Asuka returns to his home, and twelve years have passed since then. Asuka, who was grown up to be a magnificent male tiger, suddenly appears in front of Ao and asks him, "Will you be my mate as promised?" Ao is surprised by the sudden proposal from the man he once loved like a younger brother...?!

(Source: Gentosha Comics, translated)