It is generally said that 'Tobatsu (兜跋)' means Western 兜跋 country (Turfan at present), which is based on a legend that Bishamonten appeared in this place with this figure.
「兜跋」とは西域兜跋国、即ち現在のトゥルファンとする説が一般的で、ここに毘沙門天がこの姿で現れたという伝説に基づく。