Legendary enemies are enemies in Salt that are 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 engage them - therefore they are considered rare. More of same legendary can be encountered in different location and some of them do respawn from the same place you found them
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.