This animal is: from folklore, also a cryptid
The Tsuchinoko is a reptile, with venomous fangs, a thick body and thin tail (I think it looks rather like a little snake who ate a big watermelon). They are said to grow to about 80cm long when adult. It is snake-like, but has eyelids, so is more like a limbless lizard (snakes do not have eyelids). They are described as being either entirely black or mottled browns and greens with a paler belly.
The Tsuchinoko is a cryptid - a creature people have reportedly seen but we have no physical evidence of. The Japanese term for such creatures is UMA - Unidentified Mysterious Animal. With sightings reported from all over the country - except for Hokkaido and the Nansei Islands – the Tsuchinoko is one of Japan’s most famous UMAs, featuring in manga and video games.
It is also a Yokai - a Japanese supernatural creature. It is reported in folklore, and also features in various encyclopaedias like Shinano Kishōroku from the 1800s.
The Tsuchinoko is said to live in fields. When they are young they prey on insects, as they grow bigger they take bigger prey, like frogs, and eventually they may eat things the size of cats. They also like to drink sake. They are sometimes reported to talk (and often lie) or make chirping bird-like sounds.
They are thought to usually wiggle along like inchworms, but when hunting can jump up to five metres. They also like to roll down slopes, either laying sideways, or holding their tail in their mouth to become a wheel shape. There are stories of them rolling down hills after people, sometimes tripping them up or biting their legs. But they aren’t usually considered that dangerous to people.
The creature is known by many different names in different regions: nozuchi, bachihebi, tatekurikaeshi, tsuchihebi... The name used mostly is Tsuchinoko.
‘Tsuchinoko’ (which is written 槌の子 in Kanji つちのこ in hiragana and ツチノコ in katakana), can be read to mean ‘child of the earth’, ‘small hammer’, or much more commonly ‘child of hammer’. This name refers to the creature’s shape, it looks like the head of a tsuchi (a tool that can be used as a hammer, mallet or pestle) lacking a handle.
The Tsuchinoko became popular in the 1970s, after the publication of a book by Yamamoto Soseki called Nigero Tsuchinoko that recorded his and other peoples’ sightings of the creature. Soon followed more sightings, blurry photos, supposed live captures, and appearances in anime and manga. This was known as the ‘Tsuchinoko boom’.
Another such flurry of excitement occurred in the early 21st century when a farmer in a village in Okayama Prefecture found the remains of a Tsuchinoko-like creature. It got coverage in the news, and when a biologist examined the remains they said it was “probably a yamakagashi [Tiger Keelback snake, Rhabdophis tigrinus] but not a normal one.”
External links:
This article was written with some information from The Book of Yokai by Michael Dylan Foster and Shinonome Kijin.
The Tsuchinoko on Yokai.com
The Tsuchinoko’s page on the Wikipedia (Japanese version) which has some pictures and a list of reported sightings.