Berean Study Bible
They were ruled by a king, the angel of the Abyss. His name in Hebrew is Abaddon, and in Greek it is Apollyon.

King James Bible
And they had a king over them, which is the angel of the bottomless pit, whose name in the Hebrew tongue is Abaddon, but in the Greek tongue hath his name Apollyon.

American Standard Version
They have over them as king the angel of the abyss: his name in Hebrew is Abaddon, and in the Greek tongue he hath the name Apollyon.

Darby Bible Translation
They have a king over them, the angel of the abyss: his name in Hebrew, Abaddon, and in Greek he has [for] name Apollyon.

Douay-Rheims Bible
A king, the angel of the bottomless pit; whose name in Hebrew is Abaddon, and in Greek Apollyon; in Latin Exterminans,

English Revised Version
They have over them as king the angel of the abyss: his name in Hebrew is Abaddon, and in the Greek tongue he hath the name Apollyon.

Young's Literal Translation
and they have over them a king -- the messenger of the abyss -- a name is to him in Hebrew, Abaddon, and in the Greek he hath a name, Apollyon.