مَا جَاءَ إِلَّا رَاكِبًا
He came only while riding.
Rākiban is a circumstantial qualifier (ḥāl), not the subject or object. The implicit range consists of possible manners of arriving—for instance, on foot or mounted.
The statement restricts how the arrival occurred. Interpreting it as “nobody came except Zayd” would mistakenly replace a manner restriction with an agent restriction.