What does QUA mean?
More or less, “that thing taken as itself,” as opposed to in relation to something else that might immediately come to mind if the speaker didn’t make the overt statement to consider the thing in isolation.
“qua” is a Latin word that means roughly “who”, “which”, “what” or “what kind of”. According to the online dictionary, it can also mean “whatever”, “everyone who”, “all that”, or “anything that”.
To use your example, “a zebra qua pack mule” could be a zebra being used as a pack animal.
I hope I’m close to answering your question here…if not, someone else will come along soon and set us both straight 
I like Polycarp’s answer better…
Merriam Webster says:
sine qua non
A Latin expression, used in English to mean an essential or necessary quality or condition - that without which something would not be what it is.
sine= without
non= is not
From Bryan A. Garner, A Dictionary of Modern American Usage,
“qua, conj. & adv (=in the capacity of; as; in the role of), is often misused and is little needed in English. “The real occasion for the use of qua,” wrote H.W. Fowler, “occurs when a person or thing spoken of can be regarded from more than one point of view or as the holder of various coexistent functions, and a statement him (or it) is to be limited to him in one of these aspects.” Here is Fowler’s example of a justifiable use: “Qua lover he must be condemned for doing what qua citizen he would be condemned for not doing.” But as would sure work better in that sentence; and in any event, this use of qua is expecially rare in American English.”
Let me try a (I hope not tedious) hypothetical. Let’s say someone asks you, does the Chief Justice of the United States get to vote for President of the United States. The short answer to that question, of course is yes. The longer answer is, yes, but the reason is not that he’s Chief Justice, but that he’s an Amercian citizen, etc. So, you might say, “Yes, he get’s to vote for President, but qua citizen, rather than qua Chief Justice.”
If it sounds a little pretentious, that’s because it is.
What about in this sense?:
The philosopher is not interested in the question of the ethics of a socio-political entity; he is examining justice QUA justice.
Or this?:
The clamdigger is only interested in finding clams QUA-hogs.
::D&R::