This is an actual ammonite fossil with the organic material replaced with pyrite, forming an internal cast of an ammonite. This occurs naturally with various minerals in fossils, something called permineralization. Along with pyritization, other forms of permineralization include silicification and carbonate mineralization.
In order for pyrite to replace the organic matter of the ammonite, it was buried rapidly in ocean sediment that didn't contain much organic material but was high in iron sulfide, and the seawater covering it was low in oxygen. This allowed pyrite to replace the material of the ammonite shell.
If you check out the back (below), you'll see there are bonus pyritized fossils. So. Frickin. Cool.