‘squirrel’ is difficult even for a native speaker 和 ‘squirrel’ is difficult also for a native speaker 有什么区别？如果难以说明的话，请教我一下例句。

Here 'even' sounds right to me.

The second sentence sounds as if it should b:

'Squirrel' is also difficult for a native speaker.

But, that doesn't have the same meaning. It means something more like:

ネーティーブの人にとって、「squirrel」も難しいです。