Polyamory hasn’t existed throughout the ages. Non-monogamy definitely has been around in various forms for millennia. But polyamory specifically is a relatively new historical development, only a few decades old.
This is what I was suspecting, but I could not be certain, as I certainly don't have a thoroughgoing knowledge of all world histories and cultures.
I would consider it quite possible that there may have been essentially polyamorous types of relationship in other times, prior to these last few decades. Most of what is called "history" is the history of so-called Western civilization. That history doesn't include the many other cultures which were not Western civilization. And so on....
Because what we call polyamory is so deeply rooted in, and dependent upon, an ethos of equality between the persons (regardless of gender) involved, it would not likely have come into being in a patriarchal (or even matriarchal) culture, but would more likely have arisen where power and responsibility is basically equally distributed.
Defined as such, since even modern America remains a largely patriarchal culture (for example), polyamory is more likely to emerge in/as a sub-culture than within the mainstream. But it could help shift the mainstream, too. And I think it is beginning to do just that.
Polyamory
as we know it seems to have arisen out of a cultural milieu which draws a lot of influence and inspiration from the women's movement and feminism -- as well as other liberatory movements which flourished in the period from about 1955-1975. This would, of course, also include the African American Civil Rights Movement, the LGBT movement/s, "the Hippies", 60's counterculture....