"'You didn't tell me they have, uh—freaking Mogasaurs!'" -Daniela Morales, The Fate of Ten
The Hunter had knobby, translucent skin, where black blood pumping through its power-line-sized veins could be seen. It was hairless, neckless and hunched, with a flat face. Crooked fangs protruded from its lower jaw making it impossible to fully close its mouth, a steady stream of yellowish drool spilling out. It was amphibious, with gills the size of helicopter propellers. It was quadrupedal, with its hind legs bowed, its front legs more like thick gorilla arms. It had four eyes, arranged in a diamond pattern on the beast's broad forehead, and at the center of each eye, where the pupil would be, were the Loric pendants used to track the Garde. The Hunter was almost as tall as the Statue of Liberty. Sam Goode compares the Hunter to a "tarrasque", a creature from DnD.
The Fate of Ten
The Hunter is first seen by John when Ella shows him a memory of the experiment via Telepathy, however they do not know what it is yet. It was growing from a vat, as an egg. Setrákus Ra dropped the three pendants of the deceased Garde and Five's into the vat and the Hunter devoured them.
It is later spotted when Agent Walker shows John a video of the Anubis leaving Manhattan. They could see the warship drop it into the East River, the egg larger now, "about the size of Setrákus Ra's pearl-shaped getaway ship." Because of this, the government sends down a submarine to search for it.
It later awakened while John, Sam, Daniela, Agent Walker, and other FBI agents were saving Number Nine from Five. The Hunter throws the submarine from the water and eats an agent. They run from the beast but then are pulled into Ella's vision. After the vision, Nine tries to fight it. Five later helps by aiming for the eyes and flying through its skull, blinding the Hunter from hunting Garde. The Hunter is defeated by Daniela and John by using their new Legacies, turning it into stone.
Since Sam described it to be a "Tarrasque", here are a few examples of what it may have looked like.