Susan, known to the Preacher as Joshua, was the Preacher's horse in Mercy, Nevada in 1870. According to the Eleventh Doctor, he preferred the name "Susan" and wanted the Preacher to respect his life choices. The Doctor rode Susan to find Kahler-Jex's ship. (TV: A Town Called Mercy)
Susan was a mild mannered horse but he tended to swear. Only the Eleventh Doctor knew this, however.