From Essence of Purity:

Takuma Ban, only son of the Ban familly from Hokkaido, comes to Tokyo on a quest: to find a fitting wife... with the help of Meari his cow. Jumping over a school fence while carrying Meari on his back, he unexpectedly meets a girl, Yuki Samejima. After one minute, he decides that SHE'll be his bride. Unfortunately for Takuma, Yuki's older brother, Ranmaru Samejima, is completely opposed to this.

Of course, the fact that Meari seems to have developed an instant "animal" attraction to him only reinforces Ranmaru's dislike of Takuma. Takuma is determined to make Yuki his bride and that, despite Ranmaru's stern "objections". Nevertheless, when not only Kei Enjouji but also Takuma's own family start getting involved, things get a little … "chaotic".

(Note: was originally debuted as a shounen manga, but Kodaka switched publishers and began writing it as BL)