Polygender is a Multigender that is defined as having more than two genders. It could be fluid or static.
History
Polygender was first described in 1995 as having "characteristics of multiple genders, deliberately refuting the concept of only two genders."
Flag
The flag was made by deviantart user Pride-Flags on July 3rd, 2015. Blue and pink represent masculinity and femininity respectively, and those who feel partially male/female. Yellow is for genders outside the binary. Black is for those who feel partially agender. Grey is for those whose gender(s) change or fluctuate.