ACK! Giant turtles have invaded the sea!
The zaratan is one of the largest fantasy monsters ever conceived (this one is based upon arabic myths of a giant sea turtle).
The zaratan is the tortoise to end all tortoises. They can grow to be 1000 feet in diameter, and have lifetimes measured not in years, but in millenniums. They spend the vast majority of their time asleep, floating aimlessly in the sea. They resemble a small island more than they do a living creature (they will usually be host to a whole miniature ecosystem living upon their back, even trees may grow on the back of a zaratan).
The zaratan spends 99 percent of its life in a deep sleep. They usually awaken only if injured, to mate, or to communicate with another of the same species. They float along in the seas and oceans with their mouths wide open, swallowing anything that happens to drift to close (this swallowing is a reflex action, and doesn't cause the beast to awaken). Every once in a while two zaratans will float close to each other, (if they wake up), they will spend many decades in telepathic conversation, and will eventually mate if they are of the opposite sex. It takes a pair of zaratan as long as a year to mate (and this is devasting to any plant or animal life that may be living on their backs).
Many zaratan are home to small human tribes, who are often semi-aware of the true nature of their island (usually worshipping the zaratan as a god). The zaratan will tolerate these tribes as long as they do not become a nuisance (and it is rather difficult to annoy a zaratan).
It is nearly impossible to kill a zaratan, their shells are over 10 feet thick (not counting the several feet of dirt and rocks that are usually encrusted on it), their heads average 50 feet across and are also difficult to damage. Forget even attempting to damage any of their other limbs, as they are usually inside the shell at all times. If someone does manage to damage a zaratan, then the beast will retreat to the inside of its shell, and stay there for years. They will only fight if truly injured (which would require drilling into their shells, or some equally difficult task). When a zaratan does fight, it attacks with its mouth, swallowing opponents whole. There is usually enough air inside of a zaratans stomach for a swallowed man to survive until he dies from the creatures digestive juices (a long and painful death).
As a zaratan gets older it will awaken less and less, until they never wake up again at all (they still do not seem to die, no one has ever observed a dead zaratan). Eventually they end up floating up to an island, or reef, and getting stuck there and becoming a part of the scenery. This behavior seems to be similar to that of extremely old vampires, they become less and less active until they finally stop waking up completely.