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.

In truth, the gohonzon that I revealed was not revealed by any of the many tripitaka masters entering China from India or by those who went to India from China. Looking at such books as the Record of the Western Regions by Hsūan-tsung, the honzon of various temples in many states in all of India are all recorded. I have also exhaustively studied the gohonzon of Chinese temples recorded by Chinese sages coming to Japan and by wise men of Japan entering China. Regarding those in Japan, they are all recorded in the diaries of numerous temples starting with the Gangōji and Shitennōji Temples, the first Buddhist temples in Japan, and many secular books and diaries beginning with a book called Nihongi. Therefore, the gohonzon of each temple is ascertained. The gohonzon that I revealed is not among them.

Some people may have doubts about me saying, “Isn’t it correct to say that the honzon revealed by Nichiren is not based on sutras or commentaries? Isn’t this why many sages in the past did not portray it or carve it in wood?” Despite this, the honzon is based on the scriptural statements, as clear as day. Those who doubt this should investigate whether or not the scriptural base in fact exists. It is not right to criticize it just because it was not created or portrayed in the previous era.

For instance, when Śākyamuni Buddha went up to the Trāyastriṃsá Heaven in order to make obeisance for his late mother, no one in the entire world (Jambudvīpa) was aware of this. Only Venerable Maudgalyāyana knew this, but it was due to the divine power of the Buddha. Likewise, the Buddha Dharma, which exists before our very eyes, cannot be seen unless one has the capacity for perceiving it, and it cannot be spread unless the time is ripe. There is a natural reason for this. For instance, it is like the ebb and flow of the ocean tide or the waxing and waning of the moon in the sky according to time.

Now this gohonzon had been treasured by Lord Preacher Śākyamuni Buddha in his heart for 500 (million) dust-particle kalpa in the past before appearing in this world. Even after attaining Enlightenment, he did not reveal it for forty years before expounding it in the Lotus Sūtra. And even while preaching the Lotus Sūtra, the Buddha passed through most of the theoretical section without referring to it until he began preaching it in the “Beholding the Stupa of Treasures” chapter, revealing it in the truth in the “Life Span of the Buddha” chapter in the essential section, and completing it in the “Divine Power of the Buddha” and the “Transmission” chapters.

Many bodhisattvas such as Mañjuśrī of the Konjiki Sekai (Golden World), Maitreya of the Tuṣita Heaven, Avalokiteśvara of Mt. Potalaka, and Medicine King, a disciple of Sun Moon Pure Bright Virtue Buddha, eagerly volunteered to spread it, but they were not allowed to do so. Śākyamuni Buddha then stated, “These bodhisattvas, though widely known for their wisdom and wit, have not placed their faith in the Lotus Sūtra for long and their learning is not deep enough, making it difficult for them to endure the great challenges of the Latter Age of Degeneration. I have instead my treasured disciples whom I have secretly kept in the bottom of the earth since 500 million dust-particle kalpa ago. I am entrusting them with this great duty.”

Thus, the Buddha called out such bodhisattvas as Superior Practice Bodhisattva in the “Emerging from the Earth” chapter and granted to them the five characters of Myō, Hō, Ren, Ge, and Kyō, the gist of the essential section of the Lotus Sūtra, declaring:

Listen carefully! Listen carefully! This dharma should not be spread during the millennium Age of the True Dharma nor the millennium Age of the Semblance Dharma. In the beginning of the Latter Age of Degeneration, priests slandering the True Dharma will fill the world (Jambudvīpa) evoking the anger of various heavenly beings, causing a comet to appear in the sky, and unleashing a violent earthquake that will shake the great earth like a great wave of the ocean. What is more, severe droughts, huge conflagrations, deluges, storms, widespread epidemics, famines, and the horrors of war will compete with each other. At such time when all the people in the entire world don armor and carry swords and bows, when various Buddhas, bodhisattvas, and virtuous deities have become powerless, and when people all die and fall like heavy rainfall into the Hell of Incessant Suffering, the rulers will be able to save their countries and the people will be able to free themselves from calamities if they carry the great mandala of five Chinese characters with them and put faith in it. Not only will they experience peace in this life but also will be able to escape the suffering of fire in hell after death.

Now, although I am not Superior Practice Bodhisattva, I understood beforehand nearly everything about Buddhism in the Latter Age of Degeneration. Believing that it was at the discretion of Superior Practice Bodhisattva, I have devoted myself to spreading the five characters of the Lotus Sutra during these twenty years or so.

Nii-ama Gozen Gohenji, A Response to My Lady, the Younger Nun, Writings of Nichiren Shōnin, Volume 7, Followers II,
Pages 147-149

For more details on the subject of gohonzons, see also Day 28 of 100.