Behold Capybara: Fish!
Most rodents are mouse-sized, but they range up through marmots, beavers, agoutis and maras to the sheep-sized capybaras of the South American waterways. Capybaras are prized for meat, not just because of their large size but because, bizarrely, the Roman Catholic Church traditionally deemed them honorary fish for Fridays, presumably because they live in water.*
[Richard Dawkins, The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution (Houghton Mifflin, Boston, 2004), p. 182.]
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*Isaac explains that the actual distinction is that the capybara is not a four-footed and hooved land animal, like the cow, the sheep, or the horse (which is not much eaten these days, at least in the U.S.). These are the types of animal that were prohibited eating on Fridays — fish and fowl were permitted because they were created on a different day.
In: All, Common-Place Book, Food Stuff, Raised Eyebrows Dept.