The dragon and beast and false prophet of Revelation 12:1 – 13:18 respond to the drying of the Euphrates by now making nonsense croaks from their mouths. That is, as the Lord removes the nations’ sense of security, the three beasts see an opportunity to send their unclean message to the nations. To say it better, by pouring out the sixth bowl of his wrath the Lord permits the nations to be misled and further unsettled by the (incomprehensible) croaking of hellish creatures.
13 And I saw, coming out of the mouth of the dragon and out of the mouth of the beast and out of the mouth of the false prophet, three unclean spirits like frogs.