Transgender

Among humans at least, a transgender person was someone whose gender did not align with that which they were assigned at birth. The term was used as an adjective, sometimes shortened to simply "trans".

Mickey Smith had a trans friend called Sally Salter, who was part of his Bad Wolf band. (PROSE: Rose) Jack Harkness once had a transgender co-worker, Vanessa, who previously went by the name Vincent. (TV: Greeks Bearing Gifts)

Cassandra O'Brien.Δ17 also once claimed to Rose Tyler that she had once been "a little boy" on Earth, although she was notoriously prone to misusing words and knowledge from her early Earthly life. (TV: The End of the World)

Behind the scenes
In 2015, Bethany Black became the first openly trans actor to be cast in an episode of Doctor Who, series 9's Sleep No More. She played the role of 474.