Who are the locusts from the bottomless pit?
The locusts of the fifth trumpet are strange, never before seen creatures released from the bottomless pit when the angelic "star" opens it with a key given by Jesus; they are not demons but beings created by Christ to torment those who do not have the seal of God, ruled over by Apollyon, the angel of the bottomless pit.
When the fifth angel sounds, a star falls from heaven to earth and is given the key of the bottomless pit. This star is an angelic being, and since Jesus holds the keys of hell and death after His resurrection, it is He who gives this key so the pit can be opened. Once opened, thick smoke rises like the smoke of a great furnace, darkening the sun and air, and out of that smoke come the locusts.
These locusts are unlike anything ever seen on earth. Their shape is like horses prepared for battle, with crowns like gold on their heads and faces like the faces of men. They have hair like women, teeth like lions, breastplates of iron, and wings that sound like many chariots of horses running to battle. Their tails are like scorpions, with stings in them, and their power to hurt lasts five months. They are given power as scorpions have power, and they are forbidden to harm any grass, green thing, or tree, striking instead only those men who do not have the seal of God in their foreheads. They are not permitted to kill, but only to torment, so severely that men in those days will seek death and not find it, desiring to die while death flees from them.
These locusts have a king over them, the angel of the bottomless pit, called Abaddon in Hebrew and Apollyon in Greek. This shows that there is order, governmental structure, and submission even in Hell. Rather than being demons as some might suppose, these are creatures created by Christ, who created all things, to torment Satan and his angels.
Because the 144,000 are sealed by the Angel of the Lord in their foreheads, the locusts pass over them. The Church, though still on the earth at this point, does not need that same forehead sealing, since believers are already sealed by God the Father at their new birth, as shown by being anointed and sealed with the earnest of the Spirit, and sealed unto the day of redemption.
The scriptures
And the fifth angel sounded, and I saw a star fall from heaven unto the earth: and to him was given the key of the bottomless pit. And he opened the bottomless pit; and there arose a smoke out of the pit, as the smoke of a great furnace; and the sun and the air were darkened by reason of the smoke of the pit. And there came out of the smoke locusts upon the earth: and unto them was given power, as the scorpions of the earth have power. And it was commanded them that they should not hurt the grass of the earth, neither any green thing, neither any tree; but only those men which have not the seal of God in their foreheads. And to them it was given that they should not kill them, but that they should be tormented five months: and their torment was as the torment of a scorpion, when he striketh a man. And in those days shall men seek death, and shall not find it; and shall desire to die, and death shall flee from them. And the shapes of the locusts were like unto horses prepared unto battle; and on their heads were as it were crowns like gold, and their faces were as the faces of men. And they had hair as the hair of women, and their teeth were as the teeth of lions. And they had breastplates, as it were breastplates of iron; and the sound of their wings was as the sound of chariots of many horses running to battle. And they had tails like unto scorpions, and there were stings in their tails: and their power was to hurt men five months. 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.
I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death.
Labour not for the meat which perisheth, but for that meat which endureth unto everlasting life, which the Son of man shall give unto you: for him hath God the Father sealed.
Now he which stablisheth us with you in Christ, and hath anointed us, is God; Who hath also sealed us, and given the earnest of the Spirit in our hearts.
And grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.