Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Prūsa, ae, f., a town of Bithynia, near Mount Olympus, now Brussa, Plin. 5, 32, 43, § 148.
Hence, Prūsenses, ĭum, m., the inhabitants of Prusa, Plin. Ep. 10, 66, 3 al.