Hmm... just a few comments.
"neko da!"
english equivalent: "a cat!"
this makes perfect sense. this is what one would say/think to themselves, or what you would say to your friend(s) who you're with.
I agree that this would be a natural utterance in this sort of situation. If we aren't limiting ourselves to forms of da/desu, I suspect that something along the lines of "
Neko ga iru!" might also be appropriate.
And just to be nit-picky, I'd suggest "It's a cat!" as a closer English equivalent.
"neko de gozaimasu!"
english equivalent: "a(n honourable) cat!"
you wouldn't ever say this unless you were talking to a king/queen about their cat
Actually, gozaimasu isn't an honorific, rather it's neutral polite -- that is, you're not showing respect to the referent (the cat) but simply being more polite to the person you're talking to. To borrow your example, if you were talking to the emperor (or whoever) you could conceivably "introduce" your own cat with something like "
Uchi no neko de gozaimasu." (Okay, it's a bit of a far-fetched example, but you get the idea.)
"neko ga!"
english equivalent: "the cat!"
this doesn't make sense.
I'd be hesitant to write this off as completely nonsensical. Though it probably wouldn't be uttered in the given context, it sounds fine to me as a sentence fragment, and something you might hear in a case where the speaker might be (for one example) too surprised or shocked to finish the sentence. For example, if the speaker just came upon a cat lying hurt in the road (let's say that he's still alive just so this doesn't get too depressing
), you might hear something like, "
Neko ga...! Neko ga kuruma ni hikareta!"
*i'm assuming you mean wa (=ha) as in the particle は。 There is also the particle わ which is used by females at the end of the sentence.
It's worth nothing that with this sentence-final wa, you would need a form of the copula before it, e.g. "
Ara, neko-chan da wa!"
As always, I welcome any suggestions/corrections/opinions from native speakers or anyone otherwise more knowledgable than I.