Antigender is a category of "Genders that can only be defined as the opposite of an existing gender. For instance: antiboy would be the opposite of a boy. antigirl would be the opposite of a girl.". An antigirl would feel the opposite of female, but not necessarily feel male.
Similarly, ungender is a category of genders that are "Not without, but a negative; an unboy would be the negative of a boy and an ungirl would be the negative of a girl." Antigenders are under the umbrellas of nonbinary and transgender identities, as well as possibly xenogender.
specific types
Antiboy
The opposite of a boy, but not a girl.
Antigirl
The opposite of a girl, but not a boy.
Antiagender
The opposite of no gender but neither boy or girl. Similar to aporagender.
Cendgender
a gender that changes between one gender and it's antigender. the word gender can be replaced be the gender involved, for example, cendboy is when you flip between boy and antiboy. A kind of genderfluid identity.
contragender
A gender that the opposite of their assigned at birth sex but not the other binary gender.