A Ghast isn’t able to be leashed despite a Happy Ghast is. A lead can neither be attached to a Ghast that belongs to another mob.
Reproduce:
Give yourself a Ghast Spawn Egg and a Cat Spawn Egg.
Spawn a Ghast and a cat.
Try to leash the Ghast (doesn’t work)
Leash the cat and try to leash the cat to the Ghast. (latter doesn’t work)
Comments 2
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I don't really think that Ghasts should be leashable. currently, only a few mobs (and a very small number of neutral and hostile mobs) can be leashed.
https://minecraft.wiki/w/Lead#Leashing_mobs