Ovid's Fastī for June 1 relates a story about strīgēs, witches who could transform into owls and magically sap the life of infants. There seem to be two forms of this word, strīx, -gis and strīga, -ae.
But where do these forms come from? Ovid suggests a connection to strīdēre "to screech", but that would require the D to turn into a G, so it sounds like folk etymology to me.