Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Mnestheus (dissyl.), ĕi and ĕōs, m., = Μνησθεύς, a Trojan: Mnesthea vocat, Verg. A. 4, 288.
On account of the metre, also Menestheus (trisyl.): fratre Menestheo, Verg. A. 10, 129.