Pathrusim were descendants of Mizraim (i.e., Egypt) according to the genealogies in Genesis, who inhabited Pathros (i.e., Upper Egypt).
In Parry, J. H. (ed.). "10:23". Book of Jasher. Translated by Moses, Samuel. , the Pathrusim and Casluhim intermarried resulting in the Pelishtim, Azathim, Gerarim, Githim, and Ekronim.
References
Pathrusim Wikipedia(Text) CC BY-SA