The Latin word cētus (a whale or some other major sea creature) behaves peculiarly. In singular it is a normal-looking masculine cētus, but in plural it is a neuter cētē. The original Greek word is neuter in both numbers (κῆτος, κήτη). This word came up in the search for neuter animals in Latin.
How is the word cētus declined? If it was reanalyzed as a second declension masculine in Latin, I can guess how the singular endings look like. But how about plural? The ending -ē is highly irregular for a plural neuter in Latin. (The only other one I can think of is pelagē.) The accusative is certainly like the nominative, and dative and ablative should be identical. The question boils down to two forms: What are the plural genitive and plural dative/ablative forms of cētus?
Going by the Latin second declension, cētōrum and cētīs sound likely. But the Greek word seems to correspond better to the Latin third declension, so cēt(i)um and cētibus would make sense, too. Greek style plural endings besides nominative and accusative seem very rare in Latin, but perhaps that could be possible, too.
If the forms are unattested, what should they be by reasonable analogy? Whether or not perfectly justified classically, I would much like to be able to use all forms of the word.