There is a written account of the creature by a 19th century Englishman and missionary John Batchelor, who lived among the Ainu .He wrote a journal entry of an alleged actual incident concerning an apparent Akkorokamui in his book “The Ainu and their Folklore”.
In the morning, we found the whole village under a cloud. Three men, it was said, were out trying to catch swordfish, when all at once a great sea monster, with large staring eyes, appeared in front of them and proceeded to attack the boat. A desperate fight ensued. The monster was round in shape, and emitted a dark fluid and noxious odour. The three men fled in dismay, not so much indeed for fear, they say, but on account of the dreadful smell. However that may have been, they were so scared that the next morning all three refused to get up and eat; they were lying in their beds pale and trembling.
The only other information I could find was in this posting on Cryptomundo:
So Folktale or a Giant squid or Octopus? Without further information it is difficult to make an evaluation, a modern day sighting would be helpful……so if anyone out there knows of any please post a comment.