Ningyo are somewhat like mermaids in that they resemble both fish and humans. However, unlike their Western counterparts, they are traditionally thought to be hideous. Their mouths are shaped like a monkey's and are filled with small teeth like those of a fish. The scales covering their bodies are often described as being gold, although they do come in other colors as well. 

FUN FACT: In modern times, ningyo are sometimes portrayed as being more attractive due to their depiction being influenced by the West's much more beautiful mermaids.

The delicious flesh of ningyo is said to increase the lifespan of those who eat it. However, this is rarely ever done because catching a ningyo is said to bring great misfortune and natural disasters. Fishermen who catch this creature normally throw it back into the ocean as quickly as possible.

When dead ningyo wash up on the shore, they are seen as an omen of war or calamity.

Examples in various media:

Serilly (Puyo Puyo)

Feral Ningyo (Legend of the Five Rings)

Mermaidyn (Yo-kai Watch)

Lu (Lu Over the Wall)