Thalasssus

Thallasuses are absolutely colossal leviathans of the deep sea, and one of the largest species in all of Panthea, being up to 1,500 feet (457 meters) long. Thallasuses have a very unusual and unnerving appearance, they are dark green and possess a humanoid torso but instead of legs, they have a varying number of giant tentacles. Their heads are wide in shape and they have a huge mouth that is full of countless needle-like teeth going all the way to the back of their throats. and surrounding their mouths are a large and irregular number of unblinking eyes. The arms of thalassuses are long and human-like, but instead of hands they have several large tentacles, all of a thallasuses skin is covered in countless tentacles of varying size, which along with their vaguely humanoid form, terrifying mouth, and many piercing eyes, this gives them a nightmarish appearance.
Thallasses were only discovered within the last few decades by Merindian submarines. They've remained unknown for so long because they live exclusively in the deep ocean, where nobody has ever been able to reach until recently.