Last updated: 07/10/2020
Did you know Sun Wukong was among the various martial spirits that the fighters (fig. 1) of the anti-foreign Boxer Rebellion (1899-1901) channeled to gain what they believed to be superhuman fighting ability? This ritual is described by the German catholic missionary Georg Maria Stenz in his 1907 book Contributions to the Folklore of Southern Shandong (Beiträge zur Volkskunde Süd-Schantungs) (fig. 2).
On any day of the first month, [the possessing spirit of] the monkey is invited [to earth] […] In order to invite the monkey, money is collected to buy incense in the village. On that particular day, four young men, who are not allowed to be … born in the year of the dragon or tiger, are led to any temple or cemetery … There the incense candles are lit and the following prayer is spoken:
[Yī pǐ mǎ liǎng pǐ mǎ]
[Qǐng Sūn Dà Lǎoyé lái wánshuǎ]
[Yī tiáo lóng liǎng tiáo lóng]
[Qǐng Sūn Dà Lǎoyé xià tiān gōng]
One horse, two horses.
Great Venerable Lord Sun, please come and play.
One dragon, two dragons.
Great Venerable Lord Sun, please descend from Heaven and fight.
Then the four fall on their faces and remain in this position for a while. Suddenly someone flops to one side: the [spirit of the] monkey has taken hold and the young man can no longer move himself. After being carried home, lighted incense candles are held under his nose until he jumps up by himself. Once a long saber is put in his hand, he makes a scandalous display accompanied by much fanfare and cymbals. The “possessed” is constantly brandishing the saber in the air and jumping over tables and benches. If one believes the display is too scary, then one lets the incense candles extinguish and the possessed falls immediately as if lifeless to the ground. After some time you call him by his name and he wakes up slowly as if from a deep sleep (Stenz, 1907, pp. 47-49). 
Esherick (1987) notes the term “horse” from the poem was often used by boxers to refer to the possessee (pp. 56 and 62), or the human vessel that spirits command like a rider on a horse. I imagine both the horses and dragons refer to all four men who volunteer for the ritual.
I’ve written an article about the origins of Sun Wukong’s cult in Fujian province.
Today I attended Sun Wukong’s birthday celebration (the 16th day of the 8th lunar month) in Kowloon, Hongkong. I might post an article about this in the future. In the meantime, I wanted to note that, since the worship of the Monkey King appears in so many coastal provinces, it’s possible that his cult spread via sailors/merchants.
Elliott (1955/1990) describes a spirit-medium (Hokkien: tangki, 童乩; Mandarin: jitong, 乩童) initiation ceremony in Singapore with similarities to the above ritual. These include a small number of young male volunteers; a temple (where the altar is located); chanting, gongs and drums; and one of the volunteers being possessed by a deity, followed by violent movements. This suggests a widespread tradition of spirit-mediumship using similar methods:
The candidates, who may number five or six, seat themselves in a row in front of the altar. Each is given three small incense sticks to hold. they then have to meditate on the shen [神, “god”] which they wish to invoke … After appropriate cleansing ceremonies have been performed the assistants begin to chant and beat their drums and gongs. The experienced dang-ki who has been asked to participate is standing by and possibly assisting in one or another of the minor duties. This initial stage may last for an hour or two while the candidates sit with their heads bowed, waiting to become possessed … Eventually, some slightly strange effects take place in one or more of the the novices. They may shiver a little, or shake their heads. When the experienced dang-ki sees this, he seats himself in a chair beside them and prepares to enter a trance. Within a few minutes his shen has possessed him. Rising from his chair, he strikes a posture in front of the altar and waits until the novices show further signs of possession. At last one of them begins to get more violent in his movements. His head begins to node up and down, and his body sways from side to side so that assistants have to hold his chair lest it fall over. Here the experienced dang-ki intervenes. He grabs the novice by both hands and tries to drag him to his feet … As soon as the dang-ki and assistants can support him in a standing position, they lean over and try to catch the words he is muttering. From this, or from his bodily movements, they identify the shen that is possessing him. An assistant rushes to the altar and produces the stomacher  and other items of apparel appropriate to the shen they have identified. The stomacher is tied across the novice’s chest, and he is dragged up to the offering table, still reluctantly, and in a state bordering on collapse. Here he has charm water blown over him and he is given a drink” (p. 60)
1) Adapted from the original German.
2) A stomacher (dudou, 肚兜) is an embroidered bib worn on the tangki’s bare torso. It is a symbol that the tangki is “‘naked’ to the sun”, serving as a reenactment of ancient Shang-period sacrificial rain-making ceremonies (Chan, 2015, p. 5).
Chan, M. (2015). Contemporary Daoist Tangki Practice. In Oxford Handbooks Online (pp. 1-19). New York: Oxford University Press.
Retrived from https://ink.library.smu.edu.sg/soss_research/1872
Esherick, J. (1987). The origins of the Boxer Uprising. Berkeley: University of California Press.
Stenz, G. M. (1907). Beiträge zur Volkskunde Süd-Schantungs [Contributions to the Folklore of Southern Shandong]. Leipzig: R. Voigtländer.