Apparently 19th-century Japan experienced a mermaid craze that netted scientists and natural history illustrators in the excitement of the unknown. The familiar-looking mermaid in the painting above is by Keisuke Ito (1803-1901), the father of Japanese botany, and depicts her swimming alongside a real sea lion.
Far less familiar (to me, at least) are these Japanese images of demonic mermaids. The painting above is by an unknown artist of the Edo period.
This painting was by Baien Mouri (1798-1851), a prolific illustrator known for his colorful depictions of plants and animals. I found the three images above at Pink Tentacle.
Then there was the so-called Fejee Mermaid, displayed first, according to Wikipedia, by P.T. Barnum, later acquired by Harvard University's Peabody Museum of Archaeology and Ethnology, currently off display in the museum's attic. This version of the Fejee Mermaid was made of a dead baby monkey sewn to a fish tail and covered with papier-mâché.
Here's another "real" mummified Fiji mermaid.
Meanwhile, 19th-century American mermaids were as impossibly idealized as 19th-century American women.
By 1910, when Howard Pyle painted The Mermaid, Hans Christian Andersen's promise of human-mermaid matches had become part of art's lexicon.
From there, of course, all currents led to Disney.