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. (TV: A Town Called Mercy)

Soon after his regeneration, the Twelfth Doctor came across a horse and asked it to relieve it off 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 criticised 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".