You'd think that, having observed the reproductive habits of enough birds to have noticed a pattern, everyone would just assume that Branta leucopsis, a type of wild goose, was an egg-layer, despite having no idea where its breeding grounds were located. Instead various theories were proposed to explain its origins, one of which was that they developed from the crusty growths found on wood -- trees and driftwood -- along the seashore. There it would draw sustenance from tree sap and/or the ocean until covered in feathers, at which point it would fly off.
This myth dates back to at least the 11th or 12th century and was so pervasive that various religious authorities apparently felt the need to take a stance on Branta leucopsis, or the Barnacle Goose. In Ireland, some clerics declared that Barnacle Geese could be eaten on fast days when consuming any other foods, including other types of meat, would mean breaking one's fast. Pope Innocent III prohibited the consumption of Barnacle Geese during Lent, arguing that, aside from their unusual origins, they lived and behaved exactly like other, similar birds and thus were subject to the same rules. Similarly, Rabbeinu Tam, a French rabbi in the 1100s, decided that Barnacle Geese were indeed kosher and must be slaughtered in the same manner as other animals, in accordance with Jewish dietary law.
As it turns out, the Barnacle Goose breeds in the arctic, but the name, and the word barnacle, as applied to the arthropods that cement themselves to objects in intertidal zones and areas of shallow water, stuck. Though the ultimate origin of "barnacle" is unclear, the link between the goose and the sea creature is..well, a bit too illogical for me to feel right calling it "clear," but it is well-documented, and serves as an enduring testament to human imagination and the sometimes charming products of creativity and ignorance.