Social category systems – such as nationality, class, and gender – are constantly shifting. How are emergent social groups, which had previously not been societally recognized, mentally represented alongside more established groups? The growing visibility of transgender women and men in the US is an opportunity to observe this sort of cultural shift. Across three diverse methods of stereotype measurement, we assessed characteristics associated with transgender women and men and compared them to the stereotypes of their more traditional (cisgender) counterparts. In our final study, we directly assessed how people mentally position transgender groups relative to culturally well-established cisgender groups. Across these four studies, we show that mental representations of emergent gender groups are highly idiosyncratic, such that some participants extended same-gender-identity stereotypes to transgender groups (e.g., stereotyping transgender women as feminine), while others extended same-sex-assigned-at-birth stereotypes to transgender groups (e.g., stereotyping transgender women as masculine). Moreover, these differences were substantially explained by endorsement of gender essentialist beliefs.