Updated:
1. Summon one
/summon minecraft:happy_ghast ~ ~3 ~It is large, so summon it a few blocks up in open space.
2. Summon with a harness
/summon minecraft:happy_ghast ~ ~3 ~ {equipment:{body:{id:"minecraft:red_harness",count:1}}}The harness goes in the body slot. There are 16 colors; swap red for blue, white and so on. This equipment format is 1.21.5+.
3. Summon a ghastling
/summon minecraft:happy_ghast ~ ~1 ~ {Age:-24000}A negative Age makes a baby that grows up after 20 minutes. Add AgeLocked:1b to keep it small. See baby mob command.
4. Add a harness later
/item replace entity @n[type=happy_ghast] armor.body with minecraft:blue_harness@n picks the nearest entity (1.21+). Use with air to remove it.
5. Ride right away
/ride @s mount @n[type=happy_ghast]With a harness on, the rider can steer. Sneak to get off. See ride command.
6. Change flying speed
/attribute @n[type=happy_ghast] minecraft:flying_speed base set 0.1Higher is faster. Use base reset to undo.
7. Raise one from a dried ghast
/give @s minecraft:dried_ghastPlace the dried ghast in water and it slowly rehydrates into a ghastling. Feeding the ghastling snowballs makes it grow up faster.
Troubleshooting
- Cannot summon: happy ghasts need 1.21.6+.
- Harness will not go on: babies cannot wear one.
- Cannot steer: it needs a harness.
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