Greater Rift Guardians
From Diablo Wiki
Greater Rift Guardians (GRGs) are upgraded versions of the regular Nephalem Rift Guardians. So far no one has seen any new monsters serving as a GRG, but in higher level Greater Rifts these familiar Rift Guardians have showed subtle but dangerous improvements, such as much faster attack speed, or the ability to cast their spells or minion summons much more frequently.
This page will be updated once specific difference between Greater Rift Guardians and normal Rift Guardians are known. For now, refer to the normal Rift Guardians article and individual monster pages for details.