The term "Legendary Creature" is a fan moniker given to the odd forms of sea life that appear in both of the Endless Ocean games. They are differentiated from normal sea life via unique traits, oftentimes morphological aspects that make them quite distinct.
Only two species in the first Endless Ocean are considered "legendary creatures", but in Endless Ocean 2, there are many more, with most of them needing certain requirements to be met before they will even appear to the player.
Endless Ocean places the two legendary creatures it has simply at the end of the Marine Encyclopedia, and the second game places them in the Other Fauna section. In both games, they lack scientific names, the places where these would usually be denoted instead marked with a series of question marks. Of course, some of them are presumably just to be called by the same specific epithet of whatever creature they most closely resemble, but the rest are different enough that they might require further research before this can be confirmed.
In both Endless Ocean games, legendary creatures cannot be placed in the Aquarium. This is with the singular exception of the Ancient Mother in the first game, who can be placed in the Aquarium only after the player has completed their Marine Encyclopedia in its entirety; this means that all three data entries for each creature must be found before she can be placed. Legendary creatures are also unable to be shown to diving-guide clients, as they disappear during guided tours.
- Ancient Mother/White Mother - This large creature is said thought to be a new species of whale and is at the end of the story's main plot. Seen at the Great Drop-Off.
- Magu Tapa- A giant Great White Shark that appears in the Ocean's Graveyard all year round and only at night, after the player retrieves the Shark's Tooth Necklace from Knives Reef. He was replaced by Thanatos in the sequel.
- Ice Cupid - A giant Sea Angel with a unique morphology under a glow in the North Coast of Canada area, close to a hole in the ice. The player must have Oceana with them to find it.
- Ailouros Pearl - A white Box Crab found under a glow in Valka Castle's Mermaid's Ballroom.
- Grave Keeper - An abnormally large Giant Isopod with rather unique colours seen in the Twin Caves, located in the Southern Crevasse of the Zahhab Region Depths.
- Gungnir - A fast and vibrantly-colored Indo-Pacific Sailfish found in Ciceros Strait, around North Canyon in the daytime.
- Phantom - A jet-black Manta Ray with odd markings found in the Mermaid's Ballroom in Valka Castle at night.
- Apollo - A golden Ocean Sunfish found at Deep Hole at night.
- Lady Dorthea/Divine Gift - A giant and colorful Great Sturgeon found patrolling around the wreck of the Flamingo in Ciceros Strait (where a group of Great White Sharks lurk - caution is advised when searching for her).
- Golden Catfish - A golden Piraiba Catfish found at Spirit Falls in the Cortica River Upstream after the player has fed the Servant's Ring (Found in Valka Castle) to the normal catfish swimming there.
- Pelago Crocodile/Paoul Crocodile - A prehistoric species of crocodile, thought to be extinct, found at coordinates H-8 (on land) in Gatama Atoll in the daytime, after seeing it through the telescope on Nineball Island.
- King Gigide - A pure-white Spectacled Caiman found inside the temple in the Cortica River Upstream, after finding an odd item called the Caiman Relic in the Zahhab Region's Stone Castle.
- Mama Cortica - A giant Amazonian Manatee seen in Queen's Lake when it is raining in the Cortica River.
- Leo/Arthur - A large, intelligent, battle-scarred Leopard Seal that makes his home in the Iceberg Cavern.
- Snowball/Snowy - An albino Rockhopper Penguin found at the ice hole at coordinates A-4 during a snowstorm in the Weddell Sea, first encountered during a photography mission.
- Big Bobby - A huge, man-sized Emperor Penguin with brown feathers, first encountered during a mission and afterwards found on the A-4 shore area in the Weddell Sea. He disappears temporarily when snowstorms occur.
- Kraken Jr - A juvenile Giant Squid found in the Cavern of the Gods in the Altar of Isis.
- Living Fossil/Emperor- A large, uniquely-coloured Coelacanth found in the Cavern of the Gods in the Pillars of Shadow. He disappears at night and is replaced by Okeanos's Guardian.
- Cacao Maharaja/Cocoa Maharaja - A giant Leatherback Turtle found in central Gatama Atoll, during the daytime only.
- Thanatos - A huge, jet-black, dangerous Great White Shark that can oftentimes be seen patrolling the Wreck of the Emerald Lady if not purposefully going out of his way to attack the player (such as the early incident in Triton Village). He can also be seen swimming around outside of the windows of Valka Castle, but the player cannot focus on him from there.
- Black Harbinger- A jet-black Humpback Whale found in the Spring Garden of Gatama Atoll during a "Spring Tide" event - an event that occurs only on the morning or night of a new moon.
- Leviathan- An albino Sperm Whale found in the Super Drop-Off of Zahhab Region in the daytime only. Though he has a reputation for being dangerous, he poses no threat to the player.
- White Mother/Ancient Mother - A pure-white whale, thought to be a new species closely related to the Blue Whale, seen in the North Coast of Canada after receiving a mysterious Whale Ornament on the beach near the bonfire on Nineball Island.
- Singing Dragon- A new species of whale seen at the end of story's main plot. A herd of them lives in the in the Cavern of the Gods in the Pillars of Light room, seen during the daytime only.
- Okeanos's Guardian/Okeanos' Guardian - An enormous, grotesque creature, thought to be a Goblin Shark that was selectively bred by the Okeanides to guard the great treasure of the Cavern of the gods. One individual attacks the player in the final climax of the game's story. Afterwards, it is found patrolling the Pillars of Shadow during the nighttime.
- Anomalocaris- A prehistoric Arthropod, thought to be long extinct, seen swimming outside of Valka Castle's windows, where it cannot be focused on. The player gets both its basic and trivia information upon sighting it for the first time.
- Cameroceras- A prehistoric orthocone cephalopod (otherwise known as a "straight-shelled nautiloid"), seen lurking in the outer reaches of the Zahhab Region, where it cannot be focused on. The first time the player sees this creature, both its main information and trivwia are added to the encyclopedia.
- Sea Serpent- What is thought to be a prehistoric plesiosaur, seen outside or Ciceros Strait at the South Canyon, where it can't be focused on. Similarly to the Cameroceras and Anomalocaris, the player will receive all of the information on this creature upon first seeing it.
Criticisms have come from several players concerning the legendary creatures in both games. One of the main concerns is that they are not very creative, oftentimes just being resizes or recolours of other creatures, sometimes just simple albinism - easily explained, and not very unique. Others have noted that the presence of mysterious, unexplainable, or even implied-to-be-magical creatures can strike a blow to the otherwise mostly-sound scientific realism of the games. This includes the fact that one of them (the Ancient Mother) could even be placed in the player's Aquarium, despite her high status.