Shuuchaku α no Choukyou Method

Asahi and Kei are next-door neighbors who grew up together. Kei was always at his side, which Asahi believed would never change. One day, Asahi gets himself tested and is diagnosed as "a particular omega" and mentions it to his trusted best friend. Though he's an heir to a big toymaker, Kei's usually frank, down-to-earth, and a true alpha, but for some reason, a change suddenly comes over him on hearing the news, turning him into a wild animal...
(Source: Renta!)
Note: Includes 1 extra chapter.