The Humanoid Dwarf Gekko appears in five of the levels. It doesn't unlock anything within the game but it does get you an achievement for destroying all five of them in a single playthrough. What this means is that you have to play from R-02 (the appearance of the first) and play through the end of the game while completing each mission in between in succession.
The Humanoid Dwarf Gekko is a reference to a scene in Metal Gear Solid 4: Guns of the Patriots where three of these Dwarf Gekkos stood upright wearing a trench coat and fedora to pose as a person.
