The Bug
Zombie Horses & Zombie Nautilus' cannot be spawned in Peaceful Mode despite not inherently being hostile
Reproduce
Go into Peaceful mode
Get a spawn egg for a Zombie Horse or Zombie Nautilus
Try to spawn it
Observed Result
They didn’t spawn
Expected Result
They would spawn
Note
Prior to this version Zombie Horses could spawn in Peaceful Mode
Can confirm, change is also inconsistent with Skeleton Horses, which do not despawn on Peaceful Difficulty