Mofutte Ii no wa Omae dake

Determined to defeat all canines, Shiro is a cute little white cat with a selfish attitude who's unwillingly picked up off the streets by Kuro's owner. Kuro is the large black dog who vows to protect Shiro, but one day after the two are all grown up, Shiro suddenly goes into heat!

(Source: Libre, translated)