Well, of course many ancient gods and heroes had multiple wives/concubines, and some goddesses had husbands along with other lovers/consorts as well. You can find poly love in Jewish, Greek, Egyptian, Babylonian and Hindu mythology.
There is a popular contemporary book called The Red Tent, that is based on the story of Dinah, Jacob's daughter, from the Old Testament. The interplay of the multiple wives and concubines of one man is very interesting.
These ancient stories don't specifically relate to modern (a la Heinlein) polyamory, but do bear some similarities.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
In a newish relationship with Steve, (34, poly)
"Master," (mono, 34), miss pixi's Dom for 3 years