Hatsujou Usagi no Midara na Ongaeshi

"Does my repayment... feel good?" One day, Shoichiro, a novelist in a slump, saves a rabbit attacked by a stray dog. A few days later, an unknown youth comes to his house and declares that he is the rabbit that he saved. Ignoring Shoichiro's suspicions, the youth who announced himself as a rabbit boasts that he's come to repay his debt of gratitude. That night, he slipped into Shoichiro's futon and began giving him sloppy head.

(Source: Mobile Media Research)