Palaeophis - the giant sea snake
lurking just below the surface of the waters of Fjordur resides tales of sea serpents, thought to be legends of old, but these are not mere tales of mad men with sea sickness, they are actually giant prehistoric sea snakes that have survived eons, always looking and waiting for their next meal.
Wild: in the wild you can find palaeophis usually swimming in the open seas. They are usually temper mental when someone is in their territory, so it's wise to give them a wide birth. Otherwise you may become their next meal by wrapping their long serpentine body around you and secrete a highly toxic venom that they have evolved to help take down their favourite meal, eels.
Tame: once tamed these sea serpents will help aid you through the treacherous waters of Fjordur. Able to grab enemies by wrapping itself around them, it's sure to dispatch foes quickly with their powerful venom that they have evolved to take down their favourite food, which are eels that have also evolved to withstand the snakes venom, they are at an evolutionary arms race with each other.
So my idea is that they are passive tames where you have to kill and feed them eels but since they are aggressive you can only do it from behind. That or maybe just knock out tame and biotoxin as their kibble or something similar to the acro from ark additions where you have to shoot it and feed it biotoxin.
I was also thinking they can deal extra damage to those pesky eels and immune to being grabbed by squids.
Thanks to anyone reading and voting for my suggestion. I know I'm late so don't expect this to win but wanted to throw this out there.
edit: I also think this creature should be able to go on land but make slow like the opposite of a basilisk.