A novel maize variety designated X08R866 and seed, plants and plant parts thereof are produced by crossing inbred maize varieties. Methods for producing a maize plant by crossing hybrid maize variety X08R866 with another maize plant are disclosed. Methods for producing a maize plant containing in its genetic material one or more traits introgressed into X08R866 through backcrossing or genetic transformation, and to the maize seed, plant and plant part produced thereby are described. Maize variety X08R866, the seed, the plant produced from the seed, and variants, mutants, and minor modifications of maize variety X08R866 are provided. Methods for producing maize varieties derived from maize variety X08R866 and methods of using maize variety X08R866 are disclosed.