Polyanthropy is a term with two definitions. It could either be a gender neutral version of polygyny and polyandry (example: a person who marries multiple men and women, or a person who marries multiple non-binary or anonbinary people) or it could be an umbrella term for polygamy and polyamory.
The adjective, referring to the person or qualities, can be polyanthropist, polyanthrope, polyanthropic, polyanthropical, polyanthric or polyanthropous.
History
It is unknown when the term was coined, however it has been used amongst the community for a number of years.
Flag
The flag was coined by Tumblr user beyond-mogai-pride-flags on September 16th of 2018.
Etymology
Poly- comes from Greek polus ‘much’, polloi ‘many’. -anthropy from Ancient Greek ánthrōpos “human”.
Resources
https://beyond-mogai-pride-flags.tumblr.com/post/178157854100/polyanthropic-pride-flag