According to their website, Tamworth Distilling's Eau De Musc whiskey is flavored with castoreum.

For those who don't know, which is probably most of us, castoreum is a chemical secreted from a beaver's castor sac.

These sacs are located in the rear portion of the beaver.

So, basically, this whiskey is flavored with beaver butt!

You might be surprised to find out there are other places you've probably encountered this extract.  According to Wikipedia, castoreum can be found in perfumes and even other foods and beverages.