Horse

Horses were large mammals native to Earth and Tara. Humans used horses as transportation for thousands of years. (DW: The Androids of Tara, ''The King's Demons, et al.)

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)

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

On Tara, Count Grendel of Gracht carried Romana to his horse after she had hurt her ankle. Having never seen such a creature before, she 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)

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. (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 which he called Arthur. 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 learning English. He intended to inform Starkey that he was his host. (K9TV: Hound of the Korven)