I was doing a World Quest that required to kill a Rare Mob with a Buff on it to make it harder, but this was the first time I've had to kill it due to skipping rares while leveling.
You can see in the video that when the rare was killed it rewarded the item from the World Quest. Only when I went to loot the body that it contained the legendary ring.
That being said, they can be dropped from rares and I want to assume it has nothing to do with the world quest. Also, I think it's once chance per rare since the loot table is gone after the first kill.