Illusogender is when someone knows they're a certain gender, but experiences a feeling of "fakeness" present, either due to neurodivergence or due to outside pressure, such as from parents, society, etc. These feelings and/or pressures can be part of the gender experience.
Illusogender can be combined with the relevant genders one feels. For example, illusoboy, illusogirl, illusenby, etc.
Flag
The illusogender flag was designed by pride-color-schemes on Tumblr, also known as Pride-Flags on DeviantArt, while DeviantArt user FunCatty chose the colors. The flag was posted on September 27, 2015.[1] It features the famous "duck or rabbit" illusion. The illusoboy and illusogirl flags were posted on the same day.[2][3]
Disclaimer
The definition of illusogender does not inherently correlate to dysphoria, since one is only illusogender if they know for certain that they are the gender they identify as.