Kraken

Every sailor must beware of these huge beasts at sea. The kraken will attack ships which threaten it, but even those near the shore aren't safe from its tentacles, which lash out at anything that moves. Fighting these monstrosities is difficult indeed, and only the most powerful warriors emerge victorious from such a confrontation.

Kraken roam the open sea and many smaller waters.

In Ultima I and VI, it is called a giant squid, in Ultima III a man-o-war, in Ultima IV and V a squid, but they generally appear and act the same, with some notable differences:

The man-o-wars of Ultima III and squids of Ultima V are both capable of hurling poisonous ink at a distance, while those in Ultima IV gained the ability to shoot powerful lightning at range. In both Ultima VII and VII/2, only one tentacle of the kraken can be seen, and the beast can be fought from land (but it can no longer attack ships).

Trivia

 * The Kraken was to be included in the cancelled Ultima X as a singular, powerful being. Link: Ultima X Kraken