Legendary enemies are creatures/NPC's in Salt that is very rare, extremely powerful, endures more and deal more damage than any visually identical enemy. Besides that each of them carry unique names, they also are visually bigger and faster/slower than a related enemy.
Some of these Legendary creatures requires either quests or rituals to be completed before it's possible to find them or engange them,- therefore they are considered rare. Yet one or more of these enemies are not found just 1 time, which is not considered a bug, but have a fairly logical explanation behind it's/their next appearance.
Even if the player is fully armed and protected with the best outfitting, these enemies requires skill to be defeated, as they all do extremely damaging attacks. Some use ranged, some use poison and melee and some use only melee. Either way- they are all categorized as Legendary for a reason.