Barking

In addition to ΑΥ ΑΥ in Aristophanes' Wasps when the dog Λαβής enters, there is also βαύ in Fr. Choliambica Adespota 2:  

A. ⏒–⏑ ‘βαὺ βαύ’. B. καὶ κυνὸς φωνὴν ἵεις;