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Matsudodon (named after the city in the Chiba Prefecture) is an unknown seal-like creature that briefly appeared in the Edo River in 1973. The Edo River is nearly 60 kilometres in length and is a tributary of the Tone River. Matsudodon was described as being around seven feet in length, with a long tail, seal-like body, wrinkled skin, and a cat-like face. Unusually, it was said to lack flippers, and instead have legs. It would occasionally be heard making strange yelping or meowing vocalizations. 

Unlike most cryptids, matsudodon seemed to be one individual that visited the area, before leaving to go back from wherever it originated. Also notable, was that matsudodon didn't seem to try very hard to avoid contact, and was seen quite regularly.

Every sighting of Matsudodon occurred in 1973, with one of the first coming from a fisherman who reported seeing it emerge next to his boat. The creature then started to swim around him and roll in the water, before suddenly diving under. Another witness described seeing Matsudodon playing with a bag in the water, flinging it around as if using it as a toy.

One witness was walking her dog by the Edo River when she saw a hump moving across the surface. The dog started barking, and the hump started swimming towards them. It then popped its head out, showing its large eyes and cat-like face. The witness claimed it made a horrible noise like a "mix between a dog's bark and a cat's screech."

Another witness saw a large creature floating in the water on its back, almost like a sea otter. It then started swimming towards the shore and climbing up onto the rocky beach. When it noticed it was being watched, it turned around and disappeared under the water.

Many sightings of Matsudodon were by large groups of people. For instance, a crowd of onlookers on a bridge watched Matsudodon sunning itself and swimming around in the water. It would occasionally even vocalize, making loud meowing noises.

One witness, several decades after her sighting, told cryptozoologist Brent Swancer that she, as well as other people, would occasionally visit a spot on the Edo river and feed Matsudodon fish. She claimed it would get quite close to the shore and didn't seem shy or aggressive. She was confident it wasn't a seal, as it had "arms" that it could use to manipulate objects, almost like an otter. It was around seven feet long, with large, expressive eyes, a cat-like head, and had pointed ears and a long tail, unlike a seal.

One theory as to the origin of Matsudodon includes it being a misidentified otter, but this is unlikely, due to its size and pointed ears. Japanese river otters at this time were considered to be extant, although they were likely extirpated from the Edo River for decades by 1973.

Another theory is that Matsudodon was a misidentified seal that swam up the Edo River from the Tokyo bay. This has happened as recently as 2002, when a bearded seal dubbed Tama Chan, swam up the Tama River. Unlike seals, however, as mentioned, Matsudodon had pointed ears and a long tail.

In total, there were over 100 Matsudodon sightings, all occurring within less than a year. If Matusdodon was a member of an unknown species, wherever it came from is a mystery, as it hasn't been seen since.

Only one historical account that may describe another Matsudodon was in an 1834 account that describes an aggressive 16-foot long seal-like creature that killed 12 samurais near the Inba Marsh in the Chiba Prefecture. If this account is correct, it is likely not the same species as Matsudodon, as it is more than twice the size, and significantly more aggressive.

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