As I see that no one has given any suggestions of mythical beasts yet, there goes mine:
Aracne (ἀράχνη)
False player: As the story of this mythical being says, being an excellent weaver can create a shadow of the player and itself to escape, leaving behind the bait
Thread harpoon: By possessing a strong thread, it can shoot a strong line into a structure and quickly approach this
Hide: As a human spider, it can hide or take on a similar shape to the player for a few brief seconds
Running: The classic skill, which can lift your speed for a few simple seconds but as a countermeasure that has a cooldown time where your speed is slowly restored to normal
Eggs: As the mythology says, to invoke "hatchlings" to help attack briefly or to put a 1-2 second debuff of paralysis
Caronte / Carón / Χάρων
In its history is the Greek boatman who helps the souls, so it could be given a more symbolic character in the sea than on land making it a specifically sea mount
Safe Route: Turning on your lamp can make you momentarily invisible but you could not use the skill within 5 minutes (to avoid abuse)
Quick revival: As this subject was in charge of transporting souls, it would be ideal to take him as a "respawning point" but with the requirement of being more than 2 minutes at sea, thus avoiding the misuse of the skill
Soul Protection: Creates a protective field that absorbs damage for 10 seconds but consumes SP from both the summoner and the boatman
Fast speed: Increases the speed of navigation for a few seconds, in turn, the skill could go up to 25 m/s "paying" a fee that could be considered as some extra item or "souls" themselves that can be achieved in the game, the more souls you have the faster you can go for 10 seconds
Payday: Respecting the story of this character, it would be nice to have a skill where you pay the boatman to take you on a trip, I don't think it's automatic but maybe using map markers and he follows you, I don't know if it's possible, it would be quite interesting
There are many others that can be fully utilized, perhaps if they are focused they can attract many eastern MMOS players who love precisely those history features