A rider was somebody who rode a riding animal, particularly horses.
In 2369, Jean-Luc Picard explained his having a saddle onboard the USS Enterprise-D to Geordi La Forge, Deanna Troi, and Calvin Hutchinson as being "perfectly normal", given that "most serious riders do have their own saddles." Later, La Forge explained this to William T. Riker matter-of-factually after the first officer expressed his surprise that Picard had a saddle. Finally, Beverly Crusher, Riker, and Troi all explained this to Worf in the same tone. (TNG: "Starship Mine")