In 1940 Karl Landsteiner and Alexander Wiener injected rhesus macaque red blood cells into rabbits and guinea pigs, producing an antiserum that also agglutinated the red cells of most human donors. They labelled the responsible human factor Rh, for rhesus.
Within about a decade it became clear that the animal-derived antiserum and the human antibody found in maternal blood were not binding the same molecule. The animal serum was recognising what is now called the LW antigen, named for Landsteiner and Wiener, while the clinically important human factor is the D antigen of the Rh system, coded by a separate gene.
The two are distinct proteins on the red cell surface. By the time the distinction was established, Rh was in every hospital, textbook and blood bank record, and the misnomer stayed. Rhesus macaques do not carry the human D antigen at all.