Horse (language)

"Horse" was the name the Eleventh Doctor gave to the language supposedly spoken by horses. He claimed that by speaking horse, he was able to figure out that the horse the Preacher called "Joshua" was actually known as "Susan". He later appeared to carry on a longer conversation with the horse. It is left ambiguous whether the language actually exists, or if the Doctor was merely amusing himself. (TV: A Town Called Mercy)

Soon after his regeneration, the feverish and delusional Twelfth Doctor came across a horse and asked it to relieve it of "his pet". The man on the horse mistakenly thought the Doctor was talking to him (TV: Deep Breath)

After escaping a horse possessed by a Cuculus, the Doctor criticized its bad language. (PROSE: Houdini and The Space Cuckoos) The Ninth Doctor recalled Romana recommending he take an evening class to learn Horse over Mammoth, and doubted it would ever come in handy. (COMIC: Weapons of Past Destruction)

Behind the scenes

The Doctor's ability to speak with horses is a riff on his series 6-introduced power to speak "baby".