Triton is the moon of Neptune discovered by William Lassell in 1846.
Lassell found Triton on October 10, 1846, only 17 days after astronomers discovered Neptune itself. Triton remained Neptune’s only known moon for more than a century, until Gerard Kuiper discovered Nereid in 1949.
Triton is Neptune’s largest moon and is unusual because it travels in a retrograde orbit, moving opposite to Neptune’s rotation. That orbit strongly suggests Triton was captured rather than forming in place around Neptune; it may once have been a dwarf-planet-sized object from the Kuiper belt.
The moon is larger than Pluto and has a thin atmosphere dominated by nitrogen. Voyager 2 flew past Triton in 1989, revealing a geologically active world with nitrogen geysers, icy terrain, and a remarkably cold surface. Nereid, Proteus, and Larissa are different Neptunian moons, not the answer to this discovery question.