Short answers, pulled from the story.
Scientists Stephen P. Synnott and Bradford A. Smith announced the discovery of Proteus on the 7th of July 1989 after analyzing seventeen frames captured over twenty-one days. The Voyager 2 space probe took these images in June 1989, two months before its flyby of Neptune.
The International Astronomical Union officially named the satellite Proteus on the 16th of September 1991 to honor the shape-changing sea god from Greek mythology. This naming follows a convention requiring all moons of Neptune to bear names related to water deities or creatures.
Scientists measure the diameter of Proteus at roughly 400 kilometers, making it larger than Nereid but smaller than Triton. The body deviates significantly from a perfect sphere and instead resembles an irregular polyhedron featuring several slightly concave facets with relief reaching up to 80 kilometers.
The largest known feature on Proteus is Pharos, which spans between 150 and 255 kilometers in diameter and reaches depths of approximately 10 to 15 kilometers. This impact basin contains a central dome rising several kilometers above its floor and stands as the only named surface feature on the moon.
Proteus probably accreted later from debris generated during the capture of Triton rather than forming alongside Neptune. Collisions among original inner moons created a disc of rubble that re-accreted into today's moons after Triton's orbit circularized.