Hello! I've been messing with the mod quite a bit and I've noticed that setting the passengers into something like snowballs, boats or area_effect_clouds seems to cause errors and cause the mob along with it to not spawn. Is this feature not supported yet?

{
  "entity": "minecraft:zombie",
  "weight": 20,
  "first_day": 0,
  "last_day": 30,
  "nbt": "{Passengers:[{id:snowball}]}"
}

I'm currently using The-Hordes-1.19.2-1.2.0.jar in Minecraft 1.19.2 with a bunch of other mods.

The mod currently expects all passengers to be living entities, because I didn't think anyone would want to use non-living entities as passengers, but if you think they'll be useful, I don't see why it can't support them.

Oh yeah they will definitely be useful. You can make some creative mobs with them. Ex:
A zombie which has a snowball passenger, which then has a area_effect_cloud passenger that poisons you. Get too close to the zombie and you get hurt, kill the zombie and it denies an area.
Or a 'saboteur' illager which has an area_effect_cloud passenger that uses smoke particles that makes everyone near him invisible.

It'll definitely expand the options for unique mobs in raids.