Evil Eagle

Evil Eagle is the Nightmare of Eagle's Tower, the seventh dungeon in The Legend of Zelda: Link's Awakening. A huge, strange-looking eagle, Evil Eagle is used by Grim Creeper as a mount.

Strategy
Once you enter the Nightmare's room, climb up the tower to meet upon Grim Creeper again. He will summon Evil Eagle and attack you. Once Evil Eagle appears, jump up and strike it in the face if it's close enough. Alternatively, use the beam function of the Level 2 sword to attack it anywhere. However, this requires timing.

At times, it will stop and use its wings to try and blow you away. Equip the Pegasus Boots and dash in its direction to avoid this. After it takes a couple of hits, it will be defeated and you can retrieve the Organ of Evening Calm.