An Oberth-class ship that secretly tested an illegal Federation phasing cloak and was presumed destroyed in 2358, with a young William Riker among its crew.
The USS Pegasus was an Oberth-class starship commanded by Captain Erik Pressman. Ensign William Riker served aboard it on his first assignment out of Starfleet Academy. Its true mission was secret: the testing of an experimental phasing cloaking device that would let a ship both turn invisible and pass through solid matter, in breach of the Treaty of Algeron.
In 2358 the device malfunctioned and the crew mutinied against Pressman. Riker sided with the captain, and Pressman, Riker and a few others escaped before the ship was believed lost. In 2370 the Pegasus was found inside an asteroid in the Devolin system, half-merged with the rock after a phase experiment went wrong. Pressman, now an admiral, had Riker join the Enterprise-D's salvage mission while the Romulans also searched for the wreck.
After the cloak was used to free the Enterprise from the asteroid, Picard revealed the device to the Romulans and Pressman was arrested. The incident became a lasting mark on Riker's career and later framed his reflections in Star Trek: Enterprise's final episode.