Make the unit have a chance to drop an Item Set on death (can be found and configured by browsing Items Dropped On Death, iirc), then create the item set you want.
Example:
Rabbitboss has 80% chance to drop the "rabbitboss1" item set.
"Rabbitboss 1 itemset":
75%: Carrot.
25%: Rabbits Foot.
You will end up with 60% odds for Carrot, 20% for Rabbits foot and 20% for nothing.
You can ofc configure all these percentages, and add multiple item sets to one unit, iirc.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.