I have translated this passage from the Shincho-Ko ki a long time ago and last year was able to visit the place. This story sounds good and a great opportunity for a River Monsters special.
Gyuichi (Elisonas/Lamers) The Chronicle of Lord Nobunaga (pp. 96-97).
"In the middle decade of the First Month, a man called Matazaemon, a native of Fukutoku Hamlet in Ajiki Village, was walking along the embankment on a rainy evening. All of a sudden he saw a black thing, of a girth about as thick as one man could barely embrace, which rested with its trunk on the embankment while moving its head little by little across the dike toward the pond. The thing lifted its head when it heard Matazaemon approaching. Its head resembled that of a deer, and the eyes shone as bright as stars. When it stuck out its deep red tongue, it looked like an opened hand. The sight of the thing's glistening eyes and tongue scared the life out of Matazaemon. His hair stood on end, and fled to whence he came, running all the way from Hira to Onogi, where he had been staying. As Matazaemon told others about what he had seen, the story spread until it reached the ears of Lord Kazusa no Suke and
In the last decade of the First Month Nobunaga summoned this Matazaemon to interrogate him personally. Having heard what Matazaemon had to say, he issued orders to start draining the pond the next day in order to dredge up the serpent. The farmers of Hira Hamlet, Onogi Village, Takeda Five Hamlets, Ajiki Village, and Ajima Village were told to bring spades, hoes, and buckets for scooping water. On Nobunaga instructions, they lined up several hundred buckets, enclosed the Amagaike on all four sides, and scooped water for almost four hours. But once they had bailed out third of its contents out of the pond, the water level stayed the same, no matter how many more bucketfuls they took away. At this juncture, Nobunaga decided to go in the water and look for the snake himself. Clenching a dagger between his teeth, he plunged into the pond. After a while he emerged again, but he had seen absolutely nothing that looked like a serpent. Nobunaga told a man called Uzaemon, an experienced swimmer, to have a look under water as well. Uzaemon followed Nobunaga into the pond, but again nothing whatsoever was found. Nobunaga therefore went back to Kiyosu."
Tenka no tame!