Both prophecies in Revelation and Joel
tells of spirit-creatures.

Though the locusts were demons "like
unto horses", they were not actual horses.
They were spirit-beings that came out of
the Bottomless Pit.

The prophecy in Joel, refers to the non-
natural nature. Ordinary horses or locusts
can be killed. These evil creature couldn't
be wounded in Joel 2:8.

Also, they have power to climb walls, run
upon them, enter windows, which ordinary
horses can't do Joel 2:7-9.