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. Given the eccentricities of the Eleventh Doctor, of course, it is entirely possible that he was just joking, and that no such language exists. (TV: A Town Called Mercy)

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