Illegal hunting is spreading rampantly in this region, threatening endangered species.

rampantly 是副词，由形容词 rampant（猖獗的、蔓延的）加上副词后缀 -ly 构成。rampant 来自古法语，原指"爬行的、蔓延的"，引申为"不受控制的、猖獗的"。在这里形容非法捕猎活动不受控制地扩散蔓延。