Incorporating third-gender people and gods in my historical fantasy series creates exciting opportunities for inclusive worldbuilding. Several characters are middle-gendered shua, and the nomadic Skuda denote gender by occupation (warrior, priest, hearthkeeper), rather than physical characteristics.
In my fictional mythology of Kolkha, Bindi, is a middle-gendered god of rainbow-hued Twilight. Bindi embodies transitional, timeless, in-between states of liminality and endless possibility.
Bindi is a sibling of the bright and fierce Sun Woman and their brother Night Rider. These three divinities manifest the spiraling cycles of time and the dance of dusk/dawn, day, and night.
Bindi was inspired by third-gender gods of many world cultures.
The Sumerian myth, "The Creation of Man," from ~2000 BCE, lists a number of people created by the goddess Ninmah, including "the woman who cannot give birth" and "the one who has no male organ or female organ", which have been regarded as being third gender or androgynous.
The Akkadian mythical epic Atra-Hasis contains another iteration of this story, in which the sky god Enki specifically requests that Nintu create a "third-category" of third-gendered people.
Also, worship of the goddess Inanna in ancient Mesopotamia included "soothing laments" sung by third-gender priests called "gala."
In the Fourth Branch of the Mabinogion, based on ancient Welsh mythology, the semi-divine brothers Gwydion and Gilfaethwy (Gil-va-thu-ey) are transformed by their Uncle Math into a series of mated animal pairs: Gwydion becomes a stag for a year, Gelfaethwy a hind deer; then Gideon is a sow and his brother a boar; finally Gideon becomes a wolf, and Gilfaethwy a she-wolf.
Each year they mate and produce an offspring: a fawn, a piglet, and a wolf cub. After three years Math releases his nephews from their punishment. This myth can be regarded as a shamanistic initiation ritual for the two nephews.
Some of the Norse gods were capable of changing sex at will. For example Loki, the trickster god, frequently disguised himself as a woman. In one myth, he turned himself into a mare and had sex with the stallion Svaðilfari, later giving birth to Sleipnir, an eight-legged foal.
Archeologist Brit Solli suggests that the sky god Odin's practice of women's magic may connect Odin to a shamanistic cult that viewed gender transgression as a source of power.
Greek Gender bending
Apollo, an eternal beardless youth, had the most male lovers of all the Greek gods. In spite of having no male lovers himself, the love god Eros was sometimes considered patron of pederastic love between males.
Hermaphroditus, the two-sexed child of Aphrodite and Hermes, was transformed into an hermaphrodite after being attacked by a naiad (water sprite). In Ovid's Metamorphoses, the character Caeneus changes sex.
Other Transgendered Gods
Many deities change gender, manifest as different genders at different times, or combine to form androgynous or hermaphroditic beings in Hindu mythology.
Other transgender deities include the rainbow serpent god Ungud of the Indigenous Australians, and the transgender fertility god/dess Lakapati, of pre-Christian Philippines.
In addition to the Sumerian gala of Inanna, the māhū ('in the middle') of Native Hawaiin and Tahitian culture, the Tongan fakaleitī, and Samoan faʻafafine are traditional third-gendered people. Transgender shamans were common in ancient Scythia (enaree), and among the Southeast Asian shamans of Indonesia (bissu), Malay (sida-sida), and Burma (nat kadaw), and the Ibak Dayak people (manang).
The first question most people ask new parents: Is it a girl or a boy? Not: How are the parents faring? Not: What stars was your child born under? Not: What destiny has the oracle foretold?
For more on queer and transgender expression, I recommend Another Mother Tongue by Judy Grahn and Transgender Warriors by Leslie Feinberg. For an exploration of natural gender diversity read Biological Exuberance by Bruce Bagemil, Ph.D and Evolution’s Rainbow by Joan Roughgarden. All should be available at your local library--if not, request them!