Yeah, well it's a rare thing, I'm not entirely sure so don't hold me by my word, But I think it's based off the number of battles, it's not specifically for that one monster, Battle.. 105 and battle 198 may have a rare item so if you try to steal from the monsters in the 105th battle and the 198th battle the chances of getting a rare item are good because it's a random type thing. Just keep trying you'll get it. 
