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.