The Neptune moon discovered by Voyager 2 that is larger than Nereid is Proteus.
Voyager 2 found Proteus in 1989, shortly before its Neptune flyby. The moon is about 420 kilometers across, making it Neptune’s second-largest known moon after Triton. It is also the largest of Neptune’s regular, prograde satellites.
Proteus was missed by earlier Earth-based observations because it orbits relatively close to Neptune, where the planet’s reflected glare makes faint moons difficult to see. Its dark, heavily cratered surface includes Pharos, a huge impact crater more than 230 kilometers wide.
A common mix-up is confusing Proteus with Nereid, which was discovered in 1949 and is Neptune’s third-largest moon. Proteus is larger, but its irregular shape means it is not a perfectly spherical world.