Horse

The horse is large mammal native to Earth. 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), Mondas (BFA: Spare Parts) and presumably, Augea. (NSA: Sting of the Zygons)

Horses were used as mounts in jousting. (DW: The King's Demons)

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

On a minor note, "horse" sounds 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) 

Appearances

 * DW: Marco Polo
 * DW: The Reign of Terror
 * DW: The Crusade
 * DW: The Gunfighters
 * DW: The Smugglers
 * DW: The Highlanders
 * DW: The War Games
 * DW: The Time Monster
 * DW: The Masque of Mandragora
 * DW: The Visitation
 * DW: The King's Demons
 * DW: The Awakening
 * DW: Survival
 * DW: The Girl in the Fireplace
 * DW: The Pandorica Opens