The reason why gyuhi is also written as 牛皮 (cow hide) or 牛肥 (cattle fertilizer) in kanji (Chinese characters) is considered as that, gyuhi of older times was darkish and looked like cow hide as it was made from unpolished glutinous rice.
漢字で牛皮や牛肥とも表記するが、これは古い時代の求肥がもち米の玄米を用いて作られたために色が浅黒く、ウシの皮に似ていた為と言われる。