Horse

A horse was a large mammal native to the planets Earth and Tara. Humans used horses as transportation for thousands of years. (DW: The Androids of Tara, The King's Demons, et al.) Horses evolved on other planets, such as those ridden by the Cheetah People on their homeworld (DW: Survival) and the mounts augmented with cybernetic parts which were ridden by the partly cyber-converted police officers on Mondas. (BFA: Spare Parts)

The Trojan War ended when the Greek army, on the First Doctor's advice, constructed a giant wooden statue of a horse, subsequently hiding inside it and presenting it to Troy. (DW: The Myth Makers)

On Tara, Count Grendel of Gracht carried Romana to his horse after she had hurt her ankle. She had never seen such a creature before and asked "how it goes", but Grendel told her he was simply a Knight not a farrier. He gave her a ride on the horse to Castle Gracht. (DW: The Androids of Tara)

Horses were used – at least in England – as mounts in jousting tournaments. (DW: The King's Demons)

Equinoids were a species of humanoid horse (DW: Frontier in Space, RT: Perceptions) and another equine species resided on Sunset Strip. (DWA: Gangster's Paradise)

The Aquabi were a species identical to the mythical centaurs. Upon first seeing them, Donna Noble took them to be men on horseback. (AG: Pest Control)

While on board the SS Madame de Pompadour in the 51st century, the Tenth Doctor came across a horse he called Arthur, which had presumably crossed over through a time window from the 18th century. It followed him around the ship for a while, but Rose forbade him from keeping it. (DW: The Girl in the Fireplace)

In the English language, "horse" sounded similar to the word "host". This resulted in the Jixen warrior, Jixey, referring to himself as a horse when he made a mistake after just having learnt English. He intended to inform Starkey that he was his host. (K9TV: Hound of the Korven)