Is any animal neuter in Latin?

Question

The word animal itself is neuter in Latin, but at least all of the common animal species seem to be masculine or feminine (or common gender): canis, feles, equus, pardus, leo/leaena, lupus/lupa, ursus/ursa, vulpes… Is there any animal with a neuter name?

I am not looking for scientific names, but names used in ordinary speech. These two are not the same thing. An alternative formulation of the question is: Did the Romans use a neuter word for any animal?

If there are several neuter animals, which are most common or important?

This question arose from the discussion whether opossum would be a valid word for an opossum.

Answer 1

Ostreum, -i n (seashell, oyster) gave the ancients trouble.

There is also a feminine form ostrea, -ae f, and the neutral form was disparaged on the grounds that there were no neutral animal names in Latin. As wrote Charisius, Ars grammatica:

Haec ostrea feminino genere singulari numero an hoc ostreum neutrali dicendum sit quaeritur. et dicenda haec ostrea feminine singulari numero, quia ita ab eruditis non vane adnotatum est, nullius animalis speciale nomen inveniri quod neutrale sit.

The question arises whether one should say "haec ostrea" with feminine gender, singular number, or "hoc ostreum" with neutral gender. And one should say "haec ostrea" with feminine gender, singular number, because not without reason have learned men remarked that no neutral proper name of an animal exists.

But the neutral form is well attested, and some thought to solve the problem by saying that ostreum refers to the shell, whereas ostrea must be used when talking about the living being inside:

Ostrea primae declinationis fuerit, sicut Musa, feminino genere declinabitur, ut ad animal referamus; si ad testam, ostreum dicendum est neutro genere et ad secundam declinationem, ut sit huius ostrei, huic ostreo, quia dicit Varro "nullam rem animalem neutro genere declinari."

If "ostrea" is in the first declension, like "Musa", then it will be declined with feminine gender, in order to refer to the animal; if we refer to the shell, we must say "ostreum" with neuter gender and in the second declension, so that it is "huius ostrei, huic ostreo", because Varro said: "no living thing is declined with neutral gender".

(This seems to be from a commentary on the grammars of Aelius Donatus by one Cledonius, but I could not find this online, and it is usually cited as a fragment 8 of Varro, Lingua Latina.)

Christian Friedrich Neue in his Formenlehre (an influentual 19th century German treatise on Latin morphology) dryly remarks: „Doch diese Unterscheidung wird durch den Gebrauch nicht bestätigt“ (but this distinction is not borne out by usage). That page is also the source of the two quotes above and quotes several more ancient grammarians.

Answer 2

In general, words referring to animate beings were not neuter in Latin. This goes for both words referring to types of humans and words referring to types of animals. (A small number of exceptions exist.)

However, Ancient Greek seems to have had a larger number of neuter words with animate referents, in part I think because of the diminutive suffix -ιον (-ion) that could be applied to a noun of any gender to produce a derived noun which was regularly neuter (except maybe if it was a personal name). An example of a Greek diminutive noun referring to a kind of animal is σκύλιον "dogfish".

Some such words were borrowed (apparently, although I don't know how widely they really were used) into Latin, and since Greek words generally retain their gender in Latin, this produces "Latin" animal names with neuter gender.

Examples of animal names derived from Greek nouns in -ιον

• asterion < ἀστέριον
  a kind of spider.
  I'm not sure whether this is strictly a diminutive, but it looks like one. 

• rhagion < ῥαγίον, diminutive of ῥάξ
  a(nother) kind of spider

• phalangium/phalangion < φαλάγγιον, diminutive of φάλαγξ
  a kind of venomous spider

• ophidion < ὀφίδιον, diminutive of ὄφις
  a kind of fish

• phryganion < φρυγανιον, diminutive of φρύγανον
  an unknown animal species

• hemerobion < ἡμερόβιον
  a may-fly or other short-lived insect.
  I'm not sure whether ephemeron < ἐφήμερον was used the same way in Latin
  (I think this is derived from substantivization of a neuter adjective, rather than from a diminutive)

• sphingion < σφιγγίον, diminutive of σφίγξ
  a kind of ape

Not a big sample size, although the preponderance of spiders was funny to me. I doubt that any of these qualify as "names used in ordinary speech", but you can find them in Latin dictionaries.

Neuter words other than animal with general, not specific meaning

Aside from the word animal itself, there are a few other Latin words that refer to general categories of animals rather than specific animals, and are neuter. They are most often used in the plural, or if singular in form, as collectives; but there are apparently some singular uses with a countable singular sense or plural uses with a countable plural sense:

• iūmentum - a beast of burden
  L&S cites "iumento nihil opus est"
• armentum - a head of cattle for ploughing
  L&S cites "centum armenta"
• pecus, pecoris - a head of cattle
  L&S cites "inque pecus magnae subito vertare parentis"
• insectum - an insect (possibly, this should be viewed a modifier of "animal" with ellipsis of the head noun)

The usage of these terms is complicated so I'm not sure I've described it accurately, but they probably are worth mentioning in a discussion of neuter words for animals in Latin, despite their marginal relevance to your question.

Answer 3

The only exceptions to the rule "all animals must have gendered names" I can think of come with heavy qualifiers.

Cete means whales, and it is neuter, but its singular is cetus, which is masculine.

Numen is neuter, but if you insist that numina (fauns, sylphs, dryads, etc.) are not really animals, I would be hard put to argue against you.

At least numina have bodies, and so can be thought of as animal-like. Daemonia are disembodied, but as long as I'm mentioning numina, I might as well mention them too.

Virus is neuter, but the Romans certainly did not think of it as an animal, and neither do modern biologists, for that matter. I guess the only reason I mention it is that a lot of us think of it as if it were a microscopic animal, even though it isn't really.

That's it. That's my list. Not very impressive, I agree.