Horse

A horse was a large mammal native to the planet Earth. Humans used horses as transportation for thousands of years. (NSA: The Stone Rose, DW: The King's Demons, et al.)

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)

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)

Cultural references to horses
One of the longest-standing cultural references to horses on Earth was that of the "Trojan horse". At the First Doctor's behest, the Greek army constructed a giant wooden statue of a horse and presented it to Troy. Unbeknownst to the Trojans, the Greeks had secreted themselves inside the moveable statue, and sprang a decisive attack upon the Trojans once they were successfully behind Trojan lines. This move essentially ended the Trojan War in favour of the Greeks. (DW: The Myth Makers)

Some natural phenomenon, like the Horsehead Nebula and the Horseshoe Crab, were so named by humans because of their similarity to the anatomy of horses.

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)