From what I can tell, placeholder mobs are not implemented.
I think an easy way to accompish this would be to add a field to npc_type called placeholder usigned int and PHchance usigned int.
placeholder would have the ID of the mob that can spawn in its place. PHchance would be a number 1-100 indicating how likely the other mob would spawn. Can be as simple as:
Code:
if(rand(1,100) < PHchance)
spawn mob with an ID of placeholder
You could even cascade them, where a mob could have a placeholder, who is also a placeholder. In this case:
Code:
if(rand(1,100) < PHchance) {
if(mob with id of placeholder has a zero in its placeholder field, spawn this mob)
else
make a recursive call to this function
}
Something to chew on...