Donkey Sentences

Every farmer who owns a donkey beats it. The pronoun it in this sentence seems to have a clear meaning. But does it? Normally the phrase a donkey refers to some particular donkey; the indefinite article a refers to something that exists. But here its meaning is more abstract — a donkey seems to refer to a whole class of unfortunate donkeys. And in that case the pronoun it seems to have nothing to point to.

Yet most readers have no trouble understanding the sentence. How?