was, and is not, and yet is
This is from chapter 17 verse 8 of the Apocalypse and it is very significant in that it ties together Satan of chapter 20, the dragon of chapter 12, the beasts of chapter 13, and the seven heads and ten horns of chapter 17.
Revelation 17:8 (KJV)
8The beast that thou sawest was, and is not; and shall ascend out of the bottomless pit, and go into perdition: and they that dwell on the earth shall wonder, whose names were not written in the book of life from the foundation of the world, when they behold the beast that was, and is not, and yet is.
In chapter 13:12 the first beast is mortally wounded and replaced by the second one. In chapter 17 we have the beast that was, is not, and yet to come along with the five kings that were, the one that is, and the one that is yet to come. We should pay attention to any historical events that might fit this.
For more see my blog on the Apocalypse