Why is it der Prager?

Prager 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] inhabitant of Prague (male or indefinite sex)

[1] Einwohner von Prag (männlich oder unbestimmten Geschlechts)

Examples

[1] He is Prague, his wife is from Pilsenä

[1] Er ist Prager, seine Frau ist aus Pilsen.

Declension

1 Singular Plural
Nominativ der Prager die Prager
Genitiv des Pragers der Prager
Dativ dem Prager den Pragern
Akkusativ den Prager die Prager

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary