Beastboy is a kingender or xenogender defined as being a gender that is MIN or miaspec and feels large, monster-like, scary, or feels related to monsters and/or beasts of sorts. It can often be related to monsterkin but doesn't have to be.
There is absolutely no relation to the character Beast Boy from the Teen Titans franchise.
Beastgender was coined on January 12, 2020, by tumblr user ashix0trender0juice. The flag was created at the same time.
Other Beastgender identities were coined by Discord user Holly#0696 on August 15, 2021. This was when Beastboy, Beastgirl, Beastfluid, and Beastnonbinary were brought to light. Their flags were created the same date.