GW2 did this right. I believe nearly every zone had something akin to this, but a few strike my memory more. The crystal and ice dragons. The swamp zombie dragon near the last couple of levels. These guys were Massive and it took hundreds of players to take down. I know we currently have Behemoth and Odin to do this, more or less, but seriously. Once they get this whole server priority thing fixed, I hope they can start bringing out some big, nasty, challenging stuff with the potential to wipe the floor with entire squadrons of players at once.
This comes from doing some fate farming for the hell of it and I realized just then..wow, this is kind of sad.
Tell you one that comes to mind. You seen the FFXV video they showed at the TGS a few months back? The one with the enormous Iron Giant with the sword that could basically cleave through an entire building? Yeah. Lets bring that on.