Does that make the capybara legally a "fish" for the purpose of that
particular law?
Yes.
How would that affect courts trying to make determinations about what
other things not specifically enumerated are (legal) fish?
They would do their best to find a general principle. You see something similar in the efforts of Jewish legal scholars trying to apply Biblical Kosher food requirements to situations not described in the Hebrew Bible with Talmudic arguments.
In Catholic Canon law (from which this particular example is drawn), which presents the same fish on Friday conundrum, they have a somewhat easier task, because the Pope is someone who has the capacity to resolve such questions in an absolutely authoritative manner even without coming up for a consensus reason for the distinction. Historically, what happened was this:
During Lent, many Catholics deny themselves earthly pleasures to honor
Jesus's sacrifice. Abstaining from meat has nothing to do with animal
rights; instead, it's supposed to be a break from an ingredient
traditionally viewed as indulgent and luxurious. That's why beef,
pork, and poultry are considered meat but fish isn't. Fish was thought
of as a "simple" source of protein in the Middle Ages, whereas meat
from land animals was considered rich and therefore more "sinful."
As is the case with the laws of most religions, Catholics have found
creative ways around this rule over the centuries. Between the 16th
and 18th centuries, a clergyman in Venezuela wrote to the Vatican
asking if it was alright to eat capybaras during times of fasting. The
South American rodents (the largest on Earth) are certainly not fish,
but because they spend a lot of time swimming, the church decided to
classify them as such. The Vatican has also made exceptions for other
semi-aquatic mammals like beavers and muskrats. Reptiles that live in
water, like turtles and alligators, qualify as fish during Lent as
well.
In general, it is always possible to devise a rule that includes all outlier terms in a list, although in some cases that rule will be more contrived than in others.
For an example from non-religious law, the term "commodities" in which trading in future is regulated by the independent government agency known as the Commodity Futures Trading Commission (based in Chicago), includes all agricultural crops except onions. This is due to the Onion Futures Act of 1958 which was passed because there was a notorious incident in 1955 in which someone committed a massive market manipulation with the onion futures market that did massive harm to everyday people in the U.S. economy, even though, in principle, there is no good reason to regulate onion futures any differently from, for example, potato futures.
Similarly, even if two bottles of wine have contents that are chemically indistinguishable from each other (a client of mine is actually in the business of making exact chemical replicas of high priced wines using mass spectrometry and similar precision chemical tests) under U.S. law:
To be designated as a “California” wine, 100% of the grapes used in
the wine must be grown in that state. To bear a viticultural area
designation such as “Napa,” “Sonoma,” or “El Dorado County,” 85% or
more of the grapes used must be grown in the designated area.
So, it makes a factual distinction that doesn't really exist in a meaningful way from the perspective of a wine consumer.
Would an attorney be able to get any mileage out of the argument that
the capybara is not in fact a fish, as noted by leading fish experts?
No.
The best the attorney could hope for would be that there is also a capybara fish which is not as well known as the capybara mammal. If that were the case, the attorney could argue that the reference was to the capybara fish and not to the capybara mammal. But, if that isn't the case, the attorney has no leg to stand on legally. And, if there is an argument like that to be made, the legislative history of a law or regulation, regulations interpreting a statute shortly after it was adopted, or long standing practice in interpreting a law since its inception that is never authoritatively included in an authoritative legal document, could clearly rule out that interpretation.
Can a law or part of a law be struck down not for being wrong but just
for being false?
Not really.
In very rare instances, when the plain meaning of a law is absurd to the point of undermining the intent of the law expressed in other contexts the courts will interpret "and" to mean "or", or "shall" to mean "may", or "do X" to mean "don't do X" and will interpret it contrary to the plain reading on the grounds that it was a mere drafting error in the law. But this authority is virtually never used to invalidate a legal finding of fact.
Indeed, sometimes the law compels us to ignore certain facts.
For example, even if a parole eligibility model that included race as a factor was more accurate in predicting recidivism while on parole than the best available model that doesn't consider that factor, a parole eligibility model that considers race as a factor would be invalid as a matter of law.
Similarly, even if it is factually true that someone was in possession of illegal drugs, juries in a criminal prosecution for possession of those illegal drugs cannot consider that undisputedly true factual evidence if it was obtained in violation of the 4th Amendment.
The case of "fish" for the purpose of a particular law is much easier than that, however, because definitions can't be false by definition. There is no absolutely and universally true definition of any word. Instead, legal terms usually have multiple similar but not identical meanings.
The situation where a word has multiple meanings is called in linguistics and legal theory: polysemy.
Polysemy—the existence of multiple related meanings for the same word
or phrase—is a frequent phenomenon in legal and lay language. Although
polysemy sometimes arises by accident, it also can be strategic:
framers of legal rules can advance private and public interests by
assigning meanings to terms that are different from—though connected
to—the meanings that those terms carry outside the law. Understanding
the functions of polysemy can help us design more effective legal
rules and can shed light on ways in which legal actors translate
language into power.
Even if you are trying to conform to some meaningful and consistent concept in defining a legal term in a particular way, you can't really choose the correct definition until you know what the definition is seeking to distinguish from things outside the definition, i.e. the purpose of the law.
Definitions that don't match the most common sense of word in plain language are common in both legal statute and regulation drafting, and in contractual drafting.
