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By animal they refer to a person who abandons their 'humanity' and instead devolve to primal instinct. It is "animalistic" to behave in such a way. In the past beasts were terrifying to people, and many mauled people in terrifyingly ways. Many philosophers stated that humanity is what separated us from the those beasts, and to abandon ones humanity you would be no better than an animal. It is just a cultural thing that goes way back.