Thi Chi,
he was the 7th emperor (head) of Rome (the Beast)
Nero was the 5th emperor, (Augustus, Tiberius, Caligula, Claudius, Nero). But I think this fits as the beast had seven heads which "are also seven kings, five of whom have fallen, one is, the other has not yet come". It makes sense to me to say that the first five emperors, up to and including Nero,were the five who had fallen, the next emperor after the civil war Vespasian is the king who "is", and his son Titus had "yet to come". Titus only ruled for 2 years and hence "must remain only a little while".
The beast is then an eighth king which goes off to perdition. This would be Domitian, who was regarded as Nero reincarnated, and who also persecuted the Christians. So the beast that was (Nero), is not (during Vespasians reign) and "is to ascend...and go to perdition" as the eighth king Domitian.
I think this solution fits better than others which view Julius Caesar as first King, or include civil war rulers such as Galba in the list.
CF.