In volume 25, while helping Schierke protect the town of Enoch, Guts is confronted by a new fantastical enemy: an ogre. In Japanese: オーグル (ŌGURU). As Miura tended to do, he used a spelling reflecting the pronunciation of the name in its original language (French). In English, it would have been オーガ (ŌGA).
オーグル is used as furigana on the kanji 巨鬼. It's a word Miura made up for the occasion, composed of 巨 (KYO) meaning large, and 鬼 (ONI or KI), which can mean demon, ghost or even "ogre", in the sense of the traditional Japanese monster. The resulting word basically means "big demon".