A Discovery is a special Gaia unit in Age of Empires that can only found in the Scenario Editor and campaign mode of the game, such as the eponymous Discoveries scenario in the Tutorial Campaign: Ascent of Egypt. 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.
In the Age of Empires game mode in Return of Rome, a Gaia unit named Discovery is available in the Scenario Editor, but it is apparently unfinished. It has the icon of the Ruins and the sprite of the eye candy object Rugs. No flag will appear when a unit walks through it, but it is possible that a unit staying at a certain spot near the rug becomes invisible, and will remain so until it leaves that spot.
The Return of Rome version of the Discoveries scenario is named Exploration and uses Ruins in place of Discoveries.