Capybaras are classified as fish in Catholic canon law, on the grounds that they spend their lives in water. I’m guessing that a party of conquistadors was on the verge of starvation and got their priest to petition the Vatican to issue a retroactive ruling in their favour.
Fish isn't considered meat because English and Latin are slightly different languages. For hundreds of years Catholics were not allowed to eat meat on Friday. But the language of the church is Latin, and what Catholics were not allowed to eat is 'carne' which is the flesh of creatures from the land or the sky. So fish was fine.