Rocky Mountain oysters, also known as prairie oysters are bull testicles, and are considered a great culinary delicacy by those brave souls that indulge on a regular basis. Wherever cattle are raised from, Texas up to Montana, these "oysters" are served and enjoyed. The traditional cooking method is very simple; they are peeled, pounded flat, dusted with flour, salt and pepper, then deep-fried or pan-fried in oil.
Westonaprice.org has a great article entitled Rocky Mountain Oysters: Expanding on the List of Organ Meats
, which has lots of information as well as recipes in case you plan on enjoying these at home.