The devil has angels and these devils are fallen angels who have gone against God. They have joined the devil and became evil beings.

Mat 25:41  “Then He will also say to those on the left hand, ‘Depart from Me, you cursed, into the everlasting fire prepared for the devil and his angels:

Mat 4:24  Then His fame went throughout all Syria; and they brought to Him all sick people who were afflicted with various diseases and torments, and those who were demon-possessed, epileptics, and paralytics; and He healed them.

Act 19:16  Then the man in whom the evil spirit was leaped on them, overpowered them, and prevailed against them, so that they fled out of that house naked and wounded.

There are also the evil sons of God who have turned against God. They are called beasts.

Tit 1:12  One of them, a prophet of their own, said, “Cretans are always liars, evil beasts, lazy gluttons.”