رَأَيْتُ رَجُلًا حِمَارًا — I saw a man—rather, a donkey.
رَأَيْتُ: past verb with -tu as subject.
رَجُلًا: initially uttered accusative object, the antecedent.
حِمَارًا: accusative badal of correction; this is the intended object.
The example illustrates correcting a mistaken word, not describing a person as an animal.