Proteus is the second largest moon of Neptune behind the mysterious Triton. Proteus was discovered only in 1989 by the Voyager 2 spacecraft. This is unusual since Neptune has a smaller moon - Nereid - which was discovered 33 years earlier from Earth. The reason Proteus was not discovered sooner is that its surface is very dark and it orbits much closer to Neptune. Proteus has an odd box-like shape and were it even slightly more massive, its own gravity would cause it to reform itself into a sphere.
Voyager 2, NASA
<a rel="nofollow" class="external text" href="http://www.ciclops.org/view/3645/1989N1_Surface_Detail">1989N1 Surface Detail</a> (PIA00062)