In volume 16, episode 118, Guts is confronted by evil spirits who bring him face to face with his own dark desires.
Those feelings become embodied in a psychological construct called "闇の獣" (YAMI NO KEMONO) – The Beast of Darkness.
This is a pretty straightforward one. 闇 (YAMI) means darkness, 獣 (KEMONO) means a beast, as in a wild animal (deer, boar, wolf, etc.), and の connects the two words.