A Discovery is a special gaia unit in Age of Empires and are only found in the Scenario Editor and campaign mode of the game. Discoveries are in the shape of a white horse and can be controlled by multiple civilizations.
To "capture" the discovery, a unit must walk through or right next to it. Once the unit is in contact with the discovery, a flag rises spontaneously above it. The color of the flag corresponds to the color of the unit that captures the discovery.
Discoveries are theorized to be a reference to the Uffington White Horse, a chalked figure found in Oxfordshire, England, which is proposed to be several thousands of years old.