Mars

I was surprised when Larry Niven's Known Space future history included humanoid Martians. They live under the dust and water destroys them - so they use wells for burial, or for their equivalent of cremation. A protector exterminates the Martians in the Solar System by diverting an ice asteroid onto a collision course with Mars but they survive on the Map of Mars in the Great Ocean on the Ringworld.

Poul Anderson summarizes information about Mars in Is There Life On Other Worlds? (New York, 1963):

most of the surface is red-yellow desert broken by bleak scarps and ranges and meteoric craters;
the very thin atmosphere comprises nitrogen, argon, too much carbon dioxide and no oxygen;
the surface is extremely cold, receiving 43% of the radiation of Earth, although this would suffice for vision and photosynthesis;
maybe a few Terrestrial plants could live and grow there;
the polar caps seem to be water;
as on the Moon, there could be ice underground;
dark areas and "canals" change color, as if with vegetation, when the ice caps melt;
Martian plants might split oxygen from iron oxides;
plants could mean animals, even large and intelligent ones.

Suddenly, Anderson's section on Mars ends by reopening the question whether there might be intelligent Martians. However, fifty years later, no such beings have been detected so I think that their existence remains very unlikely.