Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Stymphālus, i, m., or Stymphā-lum, i, n., = Στύμφαλος, a district in Arcadia, with a town, mountain, and lake of the same name, celebrated in fable as the haunt of a species of odious birds of prey, very vexatious to the inhabitants, but which were finally destroyed by Hercules, Plin. 4, 6, 10, § 20; Suet. Vit. Ter. 5; Stat. S. 4, 6, 100; id. Th. 4, 298; Claud. Idyll. 2, 1.
Plur.: Stymphala, Lucr. 5, 30.
Hence,