Solid-phase rare earth monoxides have been recently synthesized via thin film epitaxy. However, it has been difficult to synthesize heavy rare earth monoxides owing to their severe chemical instability. In this study, rocksalt-type heavy rare earth monoxides REOs (RE = Tb, Dy, Er) were synthesized for the first time, as single-phase epitaxial thin films. The REOs were characterized by hard X-ray photoemission spectroscopy, X-ray absorption spectroscopy, resonant photoemission spectroscopy, and magnetization measurements. These REOs showed metallic electronic states with the almost localized 4f states, indicating the 4fn5d1 electronic configurations of Tb, Dy, and Er ions. X-ray magnetic circular dichroism measurements of TbO evidenced the magnetic ordering of the 4f spins below the Curie temperature (TC). The TC was evaluated to be 233 K for TbO, 142 K for DyO, and 88 K for ErO, where the TC decreased with the 4f electron number, approximately proportional to the de Gennes factor.