Basilis no Musume

Japan, 1920: Momiji, a pretty 17-year-old girl, is an orphan. The wealthy Midorikawa family adopted her years before, when she was a child. Her best friend is Basilis, a splendid mare. Momiji is in love with one of the Midorikawa sons, and the boy loves her too. But, trouble is, Momiji's adoptive father, the old Midorikawa clan head, wants to marry Momiji himself. Momiji tries to run away, but she is caught and eventually forced to marry her adoptive father.

Japan, 1997: a lot of time has passed, and all the previously mentioned characters are dead. But new characters are there, the descendants of the original ones... including a beautiful mare by the name of NeoBasilis... A new love triangle is forming between Ryuuto, Shuka and Shikata.

(Source: Ultimate Manga Guide)