This is a fossil coprolite (mineralized feces or "poop") recovered from a Miocene/Pliocene deposit in Florida's Peace River (Bone Valley Fm.). It was recovered with a lot of shark and cetacean material, so maybe it is shark or dolphin. The exact species of critter who made it is unknown. It is very solidly mineralized.
Refer to the photos. The black centimeter cube is shown for scale and is not included. You are purchasing the specimen shown. It measures approx. 3 inches in length (75mm). Your purchase will include an ID label.