Dream Beast: If you talk to this little boy in a room near Yangmei Port at night, you will know that he had a nightmare. If you go to the boatman again, there is one more choice. If you go to Crescent Island, you will find the dream beast, which will leave crescent feathers. When you go back to treat the boy, you can use the tracking function of the watch to see the dream beast, but it will run away immediately. You should catch it with black eyes, and then catch it.

Incubus: I don't know whether you are talking about the God of Nightmares or Mismagius. Nightmare gods cannot be directly captured, but can only be obtained through bugs or official activities. Bug operation is as follows:

Enter the first king's room. After closing the door, open the wizard option in the menu, select the Poké mon who can surf and let him enter the dark world through surfing. Then walk 200 steps to the right, 256 steps down, 63 steps to the left, open your backpack, use the digging device to come to the underground world, and immediately return to the ground, and you will reach the place where the nightmare god is caught. However, this BUG is extremely destructive and it is not recommended to try. If you try it, you must.

Three pillars: from the GBA version, or the official king of sending animals to wake up three pillars in platinum and capture them and pass them to pearls.

Three Birds of God: From GBA or went to Dr. Damu's house after platinum clearance (in Baidai City). After the dialogue, the three birds are activated and then swim on the map. The method of capture is the same as that of the dream god. (Note: All three birds will inhabit, so we should pay attention to restoring strength. )

I really suggest you play platinum. There are more platinum beasts than pearls. This is definitely the only way to catch these pearl beasts. If there is a better answer, I'm afraid the authorities will cause an uproar.