An Ambassador-class Federation starship commanded by Captain Walker Keel, destroyed with all hands in 2364 shortly after Keel warned Picard of a conspiracy inside Starfleet.
The USS Horatio was the command of Captain Walker Keel, one of Jean-Luc Picard's oldest friends and the man who introduced Jack Crusher to Beverly.
In the first-season episode Conspiracy, Keel summons Picard to a secret meeting on the abandoned mining planet Dytallix B, where the Horatio waits with two other ships, the Renegade and the Thomas Paine. Keel describes strange orders and suspicious deaths among senior officers.
Not long after the Enterprise leaves, it comes across a debris field: the Horatio has been destroyed, and Keel with it. The loss convinces Picard that the threat is real and sends him to Earth, where parasitic creatures are found to have taken control of Starfleet Command.