it's the Asterisk that follows after the Thief
Note that each enemy typically has a Common Steal and a Rare Steal - this boss included. In order to get the Rare Steal you need the thief ability Magpie, which is the Thief level 10 ability... soooo you'll have to quickly grind a thief to level 10 in the short time between bosses.
(yes you can steal both the Common and the Rare item during the same fight, just keep stealing until you get both, aka until the steal ability says the enemy has nothing)