What or who are Appollyon and Abaddon?

What or who are Appollyon and Abaddon?
Apollyon - The Greek name meaning "destroyer." Abaddon is a name meaning to perish. Abaddon appears only once in Revelation 9:11 in the KJV and NKJV as the angel of the bottomless pit whose Greek name was Apolloyon. In the Hebrew Bible the name Abadddon occurs six times (Job 26:6, 28:22; 31:12; Proverbs 15:11; 27:20. Psalm 88:11. The KJV, NKJV and NIV translate Abaddon as "destruction." While the NASB and RSV retain the word Abaddon.
"And they had as king over them the angel of the bottomless pit, whose name in Hebrew is Abaddon, but in Greek he has the name Apollyon" (Revelation 9:11).