Tagus III was a planet inhabited by the Taguans.
Two billion years ago, the planet was inhabited by a race of ancient Taguans known as the Imotru; the civilization thrived. During their time inhabiting the planet, some members participated in the barbaric sky diving ceremony, an event that was often attended by a young Q. In 2374, an older Q brought Jean-Luc Picard to Tagus III to observe his younger self's activities. (TNG novel: Q-Space, Last Unicorn RPG module: All Our Yesterdays: The Time Travel Sourcebook)
Over the course of the next two billion years, the Imotru became extinct, but new humanoid Taguans evolved on the planet. By the 24th century, Tagus had become one of the most important archaeological sites in the galaxy, with over 947 investigations being conducted. The first investigation took place around 20,000 years ago. (TNG novel: Q-Space)
While the Taguans sealed off the planet to foreign visitors in the mid-23rd century, the planet was still regarded with a wide amount of interest across the galaxy. In 2367, the USS Enterprise-D visited the planet and held the annual symposium of the Federation Archaeology Council aboard ship. (TNG episode: "Qpid")