frog
the practice is hinted in a centuries-old slavic folktale called babushka-lyagushka-skakushka. in this tale, a jug of milk was the preferred vehicle of choice for the babushka frog before hopping to i-know-not-where to find i-know-not-what1.
in 1854, a westchester farmer sent a letter to the new york times explaining his discovery of a frog in the milk2! while this case was seemingly an accident, frogs are naturally attracted to standing liquids, which may be how the practice was discovered long ago in other parts of the world.
later in 1876, it was noted that rural farmers in russia were still practicing the frog-in-milk method, but this was perceived as being of "rude manner"3.
however, going back to their attraction to liquids, it brings me to ponder the way ecosystems are perpetually trying to balance itself out; and within them, symbiotic relationships can form.
modern research has found that the skin of the european brown frog (Rana temporaria) produces antimicrobial peptides that prevent growth of bacteria4. while many studies are focused on how "resources" can be useful to the human species, i wonder what benefits the frogs receive from sitting in milk. contrary to the dairy in russia excerpt, maybe it is actually rude to NOT let the frogs sit in milk. thank you, frogs!
that said, i am not promoting the use of frogs in milk due to a few reasons. additionally, i'm not sure if frogs should absorb milk. i just find the quaint practice to be charming and, mostly, i just want frog to be happy.

references
dixon-kennedy, m. (1997) encyclopedia of russian and slavic myth and legend, p. 28. https://archive.org/....↩
a westchester farmer. (1854) the new york times, nov. 6, 1854. https://timesmachine.nytimes.com/...html.↩
virtue & co. (1876) the country gentleman's magazine, january to december 1876. https://books.google.com/....↩
mangoni, m. l., grazia, a. d., cappiello, f., casciaro, b., & luca, v. (2016). naturally occurring peptides from Rana temporaria: antimicrobial properties and more. current topics in medicinal chemistry, 16(1), 54–64. https://doi.org/....↩