It is disturbing to think that a dangerous wild animal is still at large in the quiet countryside.

it 引导的是to think that a dangerous wild animal is still at large in the quiet countryside
动词不定式做主语。

It 是形式主语，真正的主语是后面的动词不定式结构，因为它太长了，影响句子的平衡，所以就用it代替了，这句话是新概念英语3第一篇课文，你可以查一下哦

It是形式主语