Yes, the question is rhetorical, expecting the negative response, but no, this does not suggest Paul thinks that God is unconcerned about animals or that this Old Testament passage was originally about Apostles.
Paul was attempting to draw the reader's attention to an Old Testament passage which clearly teaches the same principle Paul was trying to teach the Corinthians, hence the rhetorical nature of his question. (Of course, such an argument would only have worked if the meaning of the OT passage was clear to both parties.) Paul's assumption was that the meaning of the OT passage would be clear enough to his readers -- unless they completely missed the point of it and took it to be about being nice to cattle!
The problem, of course, is that for many modern exegetes, it's not immediately apparent how the OT passage would offer clear support for Paul's argument! Paul was an expert in the Law and a very skilled exegete, so to grasp Paul's logic, it is helpful to first grasp the meaning of the OT passage through careful study of the Law in its original language and context. Hebrew professor Dr. Jan Verbruggen has done this, and has presented his findings in an article for the Evangelical Theological Society. (Here) His conclusion, after extensive examination of the nature of the Law, parallel ANE law codes, the Hebrew, etc. is that the statement was never about being nice to cattle -- it was about not short-changing your neighbor by weakening his animal when you rent it for working your land. In other words, it's just like all the other stipulations of the Law; it was meant to maintain justice and prohibit people from using others for personal gain to their detriment.
So Paul wasn't saying that God doesn't care about animals (cf. Matt. 6:26) or that the OT passage was about Apostles (in which case his rhetorical question would have been completely ineffective); he was making an appeal to the clear principle from the plain reading of this OT passage as support for his argument. Basically, "even the Law says you shouldn't short-change your neighbor for services rendered!".
Anecdotally, I do wonder if Paul didn't pick this particular law in order to suggest that he was a mere laborer (like an ox) and the one they were really being dishonest with was his master (God).