a boy with a cunt, aka a female-to-male (FtM) transgender/transsexual who hasn't had bottom surgery (and in this case who looks like a boy instead of a man, like some FtMs do).
A cuntboi is normally referred to as a male that has a vagina instead of a penis. The male is usually a twink and very feminine due to his parts. Cuntboys do not have breasts and are usually found From a FtM operation.
Fictional gender and an oxymoron: flat chested male who has a pussy instead of penis, and is able bear children.
This term was made up by furries. It was originally used sarcastically on alt.fan.furry for special snowflake characters who were so different that they needed their own gender. Sometimes used seriously by those who aren't aware of its origin.
Cuntboy is also occasionally used as an insult for transgendered individuals.
My fursona is a cuntboy. He is both male and female, but too special to be a herm. Don't question it.