Once in the neighboring land of India there was a great king in a country called Parthia, who loved to breed horses. He not only improved the quality of horses but also tried to transform cattle into horses. Eventually he even changed the men of Parthia into horses and rode them. The people of his own land were so grief-stricken over his actions that he began to change foreigners into horses instead.

When a merchant from a foreign country visited his country, the great king forced him to take a potion, turned him into a horse, and tied him in a stable. The merchant missed his native land and longed for his wife and children but was unable to go home without the king’s permission. Even if he were able to return home, what could he do in the form of a horse?

While he grieved over his misfortune day and night, his only son in his homeland began to prepare for a trip to look for him, since the expected date of his father’s return had passed. The son wondered, “Was my father murdered? Or is he too sick to move? How can I, his child, stay here and not go looking for my father?” His mother lamented saying, “My husband has not yet returned from abroad. What will I do if my only son goes away too and doesn’t return?” Nevertheless, the son missed his father deeply, so he went all the way to Parthia to look for him.

While staying in a small house, the master of the house told him:

What a pity! You are still very young and extraordinarily handsome. I had a son, but he went abroad and never returned. I don’t know whether he is dead or what became of him if he is still alive. When I think of my own son, it saddens me to even look at you. The reason why I feel very sorry for you is that there is a terrible development in this country. The king of this country, out of his love for horses, uses a mysterious herb. When the king forces a person to eat a slender leaf of the herb, that person will become a horse, and when the king feeds a horse with a wide leaf, the horse becomes a man. Recently the king forced a foreign merchant to eat the herb, changing him into a horse, and tied him in the first royal stable and keeps him as a treasure.

Believing that the king must have changed his father into a horse, the young man asked the master, “Do you know what kind of hair the horse has?” The master replied, “It is a chestnut horse with white dapples on its shoulders.”Hearing this, the young man devised a secret plan and entered the royal palace, stole a wide leaf herb, and fed it to the horse, which reverted to its original form as a human being.

The great king, who investigated the whole episode, was impressed by the filial act of the young man, returned the father to his son and stopped changing men into horses. Unless a child is filial, how can a child go to such lengths to go abroad searching for his father?

Venerable Maudgalyāyana saved his late mother who suffered among hungry beings, and Princes Pure Store and Pure Eyes changed the erroneous views of their father, Wonderful Adornment King. These are the fine examples of good children being the treasure of parents.

Sennichi-ama Gohenji, A Reply to Sennichi-ama, Writings of Nichiren Shōnin, Volume 7, Followers II, Pages 162-163

The glossary offers this:

Parthia (Ansoku-koku)

Ancient kingdom whose territory stretched from northwest of India to Persia. It was founded in 248 B.C. and fell in 226 A.D. The tale about the great king of this country cited by Nichiren in the “Sennichi-ama Gohenji” (70) is also found in the Hōbutsu-shū by Taira Yasuyori.