جَاءَنِي زَيْدٌ وَغُلَامُهُ رَاكِبٌ
Translation: Zayd came to me while his servant was riding.
جَاءَنِي: past verb with protective nūn and object -ī.
زَيْدٌ: nominative subject.
وَ: circumstantial wāw.
غُلَامُهُ: nominative topic; ghulām is muḍāf and -hu its genitive pronoun linking back to Zayd.
رَاكِبٌ: nominative predicate.
غُلَامُهُ رَاكِبٌ: entire nominal clause as ḥāl, occupying an accusative position.
Rākibun correctly remains nominative inside its own clause; only the whole clause has the circumstantial accusative function.