Interesting to note (because they are primarily water creatures) such animals as beavers, turtles, otters, manatees, and capybaras were all put under the "fish" title. In parts of South America, capybaras are still a regular item on the dinner table on Fridays in Lent.
I can imagine the letter about the capybaras (to the Pope) went something like this:
I have found a large tasty creature we all want to eat on Fridays. Please advise whether this is would be considered a fish. It stands several feet tall and looks like a gigantic guinea pig. It is tasty, just as fish is, and swims in the water, just like fish. The natives have taken a liking to the animal and if they don't get to eat on Fridays, they might throw a fit.
Please advise whether this huge rodent is a fish.
In The New World,
Montel Montenegro Montemayor Monterey, S.J.