Why is it der Rostocker?

Rostocker ends in -er, and most nouns ending in -er are masculine: in a German dictionary, 86% of them take der.

All nouns ending in -er
Sense 1
der

Meaning

[1] Residents of Rostock, someone who comes from Rostock

[1] Bewohner von Rostock, jemand der aus Rostock stammt

Examples

[1] My neighbor is a native of Rostock

[1] Mein Wohnungsnachbar ist ein gebürtiger Rostocker.

Declension

1 Singular Plural
Nominativ der Rostocker die Rostocker
Genitiv des Rostockers der Rostocker
Dativ dem Rostocker den Rostockern
Akkusativ den Rostocker die Rostocker

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary