Single-molecule magnets (SMMs) that can be trapped in one of the bistable magnetic states separated by an energy barrier are among the most promising candidates for high-density information storage, quantum processing and spintronics. From the very beginning {Mn12} until now, a considerable series of achievements have been made. However, the presence of fast quantum tunnelling of magnetization (QTM) in most SMMs, especially in Single-Ion Magnets (SIMs) provides a rapid relaxation route and often sets up a limit for the relaxation time. Here, we pursue the pentagonal bipyramidal symmetry to supress the QTM and present pentagonal bipyramidal Dy(III) SIMs, [Dy(Cy3PO)2(H2O)5]Cl3•(Cy3PO)•H2O•EtOH (1) and [Dy(Cy3PO)2(H2O)5]Br3•2(Cy3PO)•2H2O•2EtOH (2) (Cy3PO = tricyclohexyl phosphine oxide). Magnetic characterizations reveal their fascinating SMM properties with high energy barriers as 472(7) K for 1 and 543(2) K for 2, along with a record magnetic hysteresis temperature up to 20 K for 2. These results, combined with the ab initio calculations, offer an illuminating insight into the vast possibility and potential of what the symmetry rules can achieve in molecular magnetism.