"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)
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)
The Tenth Doctor encountered a horse which had arrived on the SS Madame de Pompadour in the 51st century through a time window from 18th century France, whom he named Arthur. Though he first asked the horse to stop following him, (TV: The Girl in the Fireplace) the Doctor later appeared to converse with Arthur as he confided his frustrations with his human companions to the horse. (COMIC: The Road To...)
After escaping a horse possessed by a Cuculus, the Eleventh Doctor criticised its bad language. (PROSE: Houdini and The Space Cuckoos)
Soon after his regeneration, the Twelfth Doctor came across a horse and asked it to relieve it of its pet. The man on the horse mistakenly thought the Doctor was talking to him. (TV: Deep Breath)
Behind the scenes[]
The Doctor's ability to speak with horses is a riff on his series 6-introduced power to speak "baby".