Long ago in Shu lived the Can Cong Emperor. Also, in the time of the Gao Xin Emperor, there lived a Lady Can; her family name is not known. When her father was robbed by some people, leaving only the horse on which she rode, she felt for her father and his inability to find food. Her mother therefore made an oath before the multitude: “If someone returns her father, they will be married to this girl.” The horse, hearing this speech, leapt up in alarm and shook itself with great haste, snapping its hobble and setting off. After several days the father was then able to ride back on the horse. From then on the horse neighed and neighed and would not accept the bit. The girl’s mother told her father about her oath, and he said: “Oaths to people are not promises to horses. How can a person be married to a different species? Having been able to solve our difficulties, its merit is indeed great, but the words of this oath cannot be put into practice.” The horse then bolted. The father grew angry and wanted to kill it, and when it ran further away, he shot it dead, drying its skin in the courtyard. The skin then kicked itself upright, wrapping up the girl and flying away. For ten nights the skin perched up a mulberry tree, and the girl transformed into a silkworm, eating mulberry leaves, making silk cocoons for human clothes and bedding. One day, the silkworm girl climbed the clouds and rose the horse, addressing her parents: “The Most High, because I did not neglect righteousness in either body or soul, has appointed me immortal attendant to the Nine Palaces. There is no return, but I will always cherish your memory.” It is customary in Shu that all Daoist temples sculpt a female figure draped in a horse skin, calling it the ‘Horse-head Maiden’, as a way of making offerings for silk production.
Anon., Huhai xinwen yijian xuzhi, 前2.76 (Tale 133):
Yuan Haowen 元好問, Chang Zhenguo 常振國 (ed), Xu Yijian zhi 續夷堅志 (Continued Records of the Listener), and Anon., Jin Xin 金心 (ed.), Huhai xinwen yijian xuzhi 湖海新聞夷堅續志 (Continuation of Records of the Listener with New Items from the Lakes and Seas) (Beijing: Zhonghua shuju, 1986).
This tale again involves the line between human and animal natures. The somewhat incoherent state of the Huhai telling, especially when compared to the Soushenji version, is notable. So is the Huhai version’s shift of responsibility and sympathy between parents (including the addition of the mother) and daughter. The more coherent, much earlier version in the Soushenji (abbreviated slightly- the last section of cross-references to horses and silk is omitted):
Gan Bao, Kenneth J. DeWoskin and J.L. Crump, Jr. (trans), In Search of the Supernatural: The Written Record (Stanford, CA: Stanford University Press, 1996), pp. 165-66:
Horse into Silkworm (14,350)
There is an old story which tells that in ancient times a man went on a long journey leaving no one at home save his daughter and a stallion which she had reared herself. Living in straitened circumstances and in a secluded place, she missed her father so much that she said to the stallion in jest: “If you find my father and bring him back for me, I’ll marry you!”
Immediately on hearing these words, that horse broke its tether and galloped off to where her father was. When the latter saw the horse, he was surprised and pleased and took him by the halter to mount him. The horse kept gazing back in the direction from whence he had come, whinnying in distress.
“Nothing has happened to this horse to make him behave so strangely. I wonder if things are not well at home?”
With that he mounted in haste and rode back. He began to take special care of the horse, which had shown such intelligence; he offered it extra fodder, which the horse refused to eat. However, every time the stallion saw the man’s daughter moving about, it would become excited and animated and rear and paw the earth. This happened many times and made the man so curious that he questioned his daughter in secret. She told him what she had said to the horse, adding, “This must be the reason.”
“Never speak of it again,” cried he, “for it will bring shame upon our family!” And you had best not go on and out as you were wont to do.” So saying, he secretly took his crossbow, slew the beast, skinned it, and hung the hide in his courtyard.
When he went a-journeying again, his daughter and a neighbor girl were playing with the hide. The daughter kicked it, crying, “You were nothing but a beast of burden, yet you thought to wed a human! You brought this death upon yourself, so you should feel no resentment!”
As she spoke, the hide rose up, wrapped itself around the daughter and galloped off. The neighbor girl was so frightened she could not lift a hand to help her friend, but fled and told the girl’s father. He returned to seek traces of the pair, but they had already disappeared.
After several days the girl and the horsehide were found bound together among the limbs of a tree where they had become a silkworm spinning itself a cocoon. This cocoon was large in diameter and length– very different from the ordinary kind. The women of the neighborhood gathered this kind of chrysalis and reared the worms to gain many times the profit they turned before.
Because of this story, people named the tree on which the girl and the horsehide were found, the sang tree [mulberry] because sang means lost. Everyone now cultivates this kind of tree, and the silkworm of today is descended from that first ancient cocoon.
Gan Bao 干寶, Soushenji 搜神記 (In Search of the Supernatural: The Written Record) (Beijing: Zhonghua shuju, 1979), 14.172-73 (350):
舊說，太古之時，有大人遠征，家無餘人，唯有一女。牡馬一匹，女親養之。窮居幽處，思念其父，乃戲馬曰：「爾能為我迎得父還，吾將嫁汝。」馬既承此言，乃絕韁而去，徑至父所。父見馬驚喜，因取而乘之。馬望所自來，悲鳴不已。父曰：「此馬無事如此，我家得無有姑乎？」亟乘以歸。為畜生有非常之情，故厚加芻養。馬不肯食。每見女出入，輒喜怒奮擊。如此非一。父怪之，密以問女。女具以告父，必為是姑。父曰：「勿言，恐辱家門。且莫出入。」於是伏弩射殺之，暴皮于庭。父行，女與鄰女於皮所戲，以足蹙之曰：「汝是畜  生，而欲取人為婦耶？招此屠剥，如何自苦？」言未及竟，馬皮蹷然而起，卷女以行。隣女忙怕，不敢救之。走告其父。父還，求索，已出失之。後經數日，得於大樹枝間，女及馬皮，盡化為蠶，而績於樹上。其蠒綸理厚大，異於常蠶。鄰婦取而養之，其收數倍。因名其樹曰「桑」。桑者，喪也。由斯百姓競種之，今世所養是也。言桑蠶者，是古蠶之餘類也。
Another version of the story can also be found here: https://widowcranky.com/2017/11/19/chinese-unicorn-artist-unknown/, described as ‘the silkworm girl’. The tale is analysed in detail in Miller, Alan L., ‘The Woman Who Married a Horse: Five Ways of Looking at a Chinese Folktale’, Asian Folklore Studies, 54 (1995): 275-305 (available via JSTOR here: https://www.jstor.org/stable/1178945).