User blog comment:Omega natsu2/Chapter 373: Let Live Or Let Die, Predictions./@comment-177.64.164.157-20140223231432/@comment-7418318-20140225095510

Rusty could manifest whatever he imagined. If he imagined this arm as a beast-arm it was indeed a beast-arm. He might have called it sword, but then he also called the heart or soul "fragment" so we can't judge it by words.