Season 2 Episode 18 - Fearful Symmetry 

Invisible 动物 are rampaging through town, destroying property and trampling a federal employee. On arriving in town, the agents begin their 搜索 at a nearby zoo with the distinction of never being the site of live birth.
密码: x-files, mulder, scully, full episode, season 2, episode 18, fearful symmetry, 2x18