Some ponies think Diamond Dogs hoard gemstones for the same reasons ponies do. Others think they eat them, like dragons. If ponies knew the truth, they might not be so eager to rob them of their subterranean treasure. If only they knew.
Twilight Sparkle was Celestia's chosen heir, and under her rule Equestria was destined to prosper. But then her friends passed, as mortal ponies always do, and she was left to rule alone. The years were not kind to her after that.