You're Released From the Collar.

Mitsuru has a pheromone that causes male dogs to go into rut, and Hachi is the young man who was "raised" by him. Mitsuru is the heir to a special dog training center, and suddenly a man with a muzzle on—Hachi—is brought to him. He looks completely human, but for some reason he is trained as a "military dog." Mitsuru has been attacked by dogs since he was a child because of his genetic make-up, but he easily forgives Hachi, who as obedient to his master as a dog but is also a human. However, there is a reason why Hachi has been trained as a "dog." Will his dog want to mate with him as an animal, or will he want to have loving sex with him?