Horse

Horses were large mammal native to Earth and Tara. Humans used horses as transportation for thousands of years prior to the invention of motorised transport.

The Trojan War ended when the Greek army, on the First Doctor's advice, constructed a giant wooden statue of a horse, hid inside it and presented it to Troy. (DW: The Myth Makers) The Trojan Horse would go on to become a frequently used metaphor for hidden danger.

Horses are also known to have evolved on other planets, such as Cheetah World (DW: Survival) and Mondas. (BFA: Spare Parts)

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

In England, horses were used 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. (DWAM: 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) horses where used in war. While aboard the SS Madame de Pompadour in the 51st century, the Tenth Doctor came across a horse which he nicknamed Arthur. It followed him around the ship for awhile 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 learning English, he intended to inform Starkey that he was his host. (K9TV: Hound of the Korven)