A lifelike miniature model of a mermaid floating in a sealed jar, meant as an amusing toy or circus attraction, to create the illusion that a real mermaid has been captured. It is primarily an anachronism dating from early European colonialism and its sea-spanning knick-knack industries, and has since acquired vaguely occult connotations.
Sometimes the phrase is borrowed by clever marketers, such as for items in restaurant menus.
Robert loved the museum: the frail mummies and immense sarcophagi; the stuffed platypus and pickled mermaid; the wall bristling with elephant tusks and moose antlers and unicorn horns. -- "Stories of Your Life and Others" by Ted Chiang