Susan (A Town Called Mercy)

Susan, known to the Preacher as Joshua, was the Preacher's horse in Mercy, Nevada in 1870. According to the Eleventh Doctor, the horse preferred the name "Susan" and wanted the Preacher to "respect his life choices". The Doctor rode Susan to find Kahler-Jex' ship. (TV: A Town Called Mercy)

Personality
Susan was a mild mannered horse but tended to swear. Only the Eleventh Doctor knew this, however. (TV: A Town Called Mercy)

Behind the scenes
Though the horse's "life choices" are left vague and open to multiple interpretations, many fans have interpreted his line as meaning Susan is transgender, and identifies as female. Many see this, in part because of the Doctor's use of he/him pronouns, as transphobic. In the real world, trans women tend to use she/her pronouns, in line with their gender identity, and referring to a woman as "he" would indeed be.

Toby Whithouse, who wrote A Town Called Mercy, also made a passing reference to a trans person in another of his scripts, Greeks Bearing Gifts. Jack Harkness speaks of his friend who disappeared for a while: "he comes back, and we've got to start calling him Vanessa." Whithouse uses masculine pronouns in both scripts for presumably trans female characters.