Amabie is merman-like creature with three legs. It is one of yokai, popularised supernatural spirits in Japanese folklore.
Its oldest reference is kawaraban print dated in 1846 (Koka 3) at the end of Edo period.
Image from wiki : https://en.wikipedia.org/wiki/File:Higo_Amabie.jpg
肥後国海中江毎夜光物出る
所之役人行見るに づのごとく者現す
私は海中に住 アマビヱと申す者也
當年より６カ年の間 諸国豊作也
併病流行 早々私を写し 人々に見せ候えと申して
海中へ入りけり
右写し候 役人より江戸に申し来たる写し也
Something lighten appears every evening, in Higo province.
Officer went and confirmed that someone like this picture appears.
"I am Amabie, living in the sea. From this year for 6 years, you will have good harvest in many prinvinces. At the same time, you will be visit by plague. You copy my picture sooner and show it to people.", said Amabie, and he went back to the sea.
The following picture is its copy of the officer's messenger coming to Edo.
Now we can find many amabie pictures with hashtug #amabiechallenge
The ministry of Health used its image to Stop Kansen Kakudai Covid-19 campaign.
Amabie is one of form of visiting gods in Japanese folklore. Supernatural sprits come from another world, which is represented by sea, ocean, forest, and mountain.