Kuroki Kemono wa Ai ni Otsu

When thief Karim sneaks into a clandestine auction, he finds Asena, a beautiful young man with animal ears and a tail. On impulse, Karim steals him from the highest bidder and decides to take responsibility for him, making arrangements to live with him for a month while he waits for the ship to his hometown to arrive.

Asena is proud and fiercely defensive of his own species, so he treats Karim – a human being – coldly, but one day he suddenly changes tact and tries to seduce Karim. It seems Asena periodically goes into heat, and Karim can't resist!

(Source: Tokyopop)