In isolation, زَيْدْ can end in sukūn when stopping.
جَاءَ زَيْدٌ. — Zayd came: subject (fāʿil), nominative, ḍamma.
رَأَيْتُ زَيْدًا. — I saw Zayd: object, accusative, fatḥa.
مَرَرْتُ بِزَيْدٍ. — I passed by Zayd: genitive, kasra.
The changing endings demonstrate that the noun itself is declinable.
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The indeclinable noun: definition, rule and eight classes
In isolation, زَيْدْ can end in sukūn when stopping.
جَاءَ زَيْدٌ. — Zayd came: subject (fāʿil), nominative, ḍamma.
رَأَيْتُ زَيْدًا. — I saw Zayd: object, accusative, fatḥa.
مَرَرْتُ بِزَيْدٍ. — I passed by Zayd: genitive, kasra.
The changing endings demonstrate that the noun itself is declinable.