Duplicate the monster object to create the flying monster.
Change the name and sprite (to the right facing flying monster).
Then go into the End Step event and change the sprites to the flying monsters.
Go into your room and place a few of the flying monsters in the air. Use markers to limit how far across the monsters can go.
- have some monsters going across the whole room (near the top, by the clouds)
- have some monsters only going across a small section or half of the room
- markers can be in the bottom square row of the monster's body, or the top square row