Gods in Human Form
For thousands of years Gods weren’t in human form. Then they seem to have been combined with animals, and then finally became completely human. I think it is our way of trying to understand both them, and our place in the world.
Our more recent myths are very human, with the Gods in actual human form, even though they retain some very Otherworldly attributes. But these are held within myth, and to me the myth is the conduit for a hidden message, not a tale to be taken literally.
So Rhiannon in her story is a woman from Annwn, who rides a strange horse, marries Pwyll, her son is taken, and she is blamed, she is sentenced to carry people on her back, then the child is returned on the night of Beltane when a great monster keeps stealing young horses, and the story continues. The story is entertaining, it is enthralling, and memorable, and tells of a great Goddess, but I don’t think she really married Pwyll, or did any of those things as a human, but the story does tell of the nature of the Horse, of the Spirit of that animal, of the Spirit called Rhiannon.
Gwydion is a git in the myths, but without him nothing would change, the tale and message would stop, so in the myth Gwydion represents Chaos, the causer of change, which is difficult, often unwanted, but also often the bringer of new thoughts, ideas, direction.
Great forces, great powers with their own intelligence, given human form in an effort to understand them. IMO that is.