A: Boxgrove, Chichester, England, United Kingdom
A fragment of a horse shoulder blade discovered by a team led by Mark Roberts at Boxgrove, England "contains a semicircular wound made by a weapon such as a spear, indicating it was killed by early humans. Other horse bones from the same site have butchery marks from stone tools." (Smitsonian, What does it mean to be human?)
For many thousands of years before horses were ridden and domesticated people killed horses for food.