na-adjective, noun
Formal or literary term describing animals, people, or actions that are violently aggressive and intimidating.
その森には凶猛な獣が住んでいる。
Fierce beasts live in that forest.
彼の凶猛な性格は周囲を震え上がらせた。
His ferocious personality made those around him tremble.
Compound of 凶 (kyō, 'evil, bad') and 猛 (mō, 'fierce, violent'). The spelling 兇猛 uses the variant 兇, which carries a similar meaning of wickedness.