ettin/ADD/<katana>

Ettin being the creature type, ADD/<katana> is the item type.

You can also name the item or add properties to it within the brackets

Such as

ettin/ADD/<katana/name/Shiny sword>
 
A quick google turned this up on these forums

ADD keyword allows you to add items to spawning mobs packs. The syntax for ADD is /ADD/itemtype, or /ADD,probability/itemtype, or /ADD,probability/<itemtype/prop1/value/prop2/value...> where probability is a drop probability between 0 and 1 and <itemspec> is an embedded item specification, so that you can put attributes on the added item as well. An example of this would be the specification string ogre/name/Lord Bleck/ADD,0.3/katana that would spawn an ogre named Lord Bleck with a 30% probability of dropping a katana.
 
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