Mountain Oyster (or Prairie Oyster)

“Mountain oysters” (also called “Rocky Mountain oysters” or “prairie oysters” or “calf fries,” among many other names) are the testicles of animals, deep-fried. This part of an animal was sometimes thrown away, but it became into culinary use in the second half of the 19th century. “Calf fries” became popular at the Fort Worth Stockyards from the 1920s.
   
   
Wikipedia: Rocky Mountain oysters
Rocky Mountain oysters, mountain oysters, prairie oysters, Montana tendergroin or swinging sirloin are North American culinary names given to buffalo, boar or bull testicles. They are usually peeled, coated in flour, pepper and salt, sometimes pounded flat, then deep-fried.
 
It is a well-known novelty dish in parts of the American West and the Canadian Prairies where cattle ranching is prevalent. The name is derived from the Rocky Mountains. In Oklahoma and North Texas, they are sometimes called calf fries but only if taken from very young bulls. In many parts of Mexico they are referred to as “criadillas” and are colloquially referred to as huevos del toro (literally, “bull’s eggs” but huevos is also a Spanish slang term for testicles) in Central and South America. Rocky Mountain oysters are sometimes confused with lamb fries or animelles (lamb testicles), which are served in a manner similar to Rocky Mountain oysters. Boar (hog) testicles are served in some Midwest areas such as in Illinois, Indiana, and Iowa. In other countries, testicles are known as sweet meats.
 
Texas Cooking
Texas History from the Dinner Table
by John Raven, Ph. B.
(...)
I suppose everyone has heard of Rocky Mountain Oysters or Mountain Oysters or Calf Fries or Turkey Fries. These are the testicles of cattle, hogs, sheep and even turkeys. At the spring roundups on the prairies of Texas, the bull calves are treated to a simple operation that prevents them from ever being fathers. After this surgery, they are known as steers. In the old days, the testicles were thrown in the fire used for heating the branding irons until they were done (the testicles—not the branding irons). Then they were skinned and eaten as is.
 
Today the preferred method of preparing mountain oysters is to soak them in cold salt water for a while, then remove the skin and slice into manageable size, dredge in seasoned cornmeal or flour and deep fry. Lots of folks swear by them. Personally, they taste to me like fried lard. Calf fries and turkey fries are given the same treatment. Incidentally, the turkey doesn’t survive the operation. Turkey fries are removed when they are dressed. 
 
Cooks.com
TEXAS CALF FRIES OR MOUNTAIN OYSTERS  
Calf fries as many as you want. Hot, not boiling water. Salt and pepper. Dip calf fries into hot but not boiling water. Leave 1 to 2 minutes. Longer if frozen. Drain and peel off outer membrane. Cut into bite size chunks. Beat together 1 egg, 1 cup milk, dip fries in liquid, then in equal parts of flour and cornmeal that has been seasoned with salt and pepper. Fry in deep, hot shortening. When they are done they will float to the top. Drain on paper towels; serve warm with dip. 
 
(Oxford English Dictionary)
mountain oyster n. the testicles of a calf, sheep, or other animal, used as food; lamb’s fry; cf. PRAIRIE OYSTER n. 2.
1857 Washington Star 21 July 2/2 Entrees... Vol au vent, a la financier; young chickens, Maryland style; *mountain oysters, sauce royale [etc.].
1937 A. WYNN in J. F. Dobie & M. C. Boatright Straight Texas 217 At branding time there was that delicacy known as the mountain oyster.
1951 E. PAUL Springtime in Paris xi. 189, I have consumed mountain oysters and prairie dancers that are actually poetic.
1998 G. VIDAL Smithsonian Inst. i. 18 I’m about to have me a pair of mountain oysters.
 
(Oxford English Dictionary)
prairie oyster, n.
A calf’s testicle, cooked and eaten as a delicacy.
1941 Amer. Speech 16 181 English slang metaphor also has its place, as..prairie oyster for the testicles of a steer, a food morsel considered dainty.
1955 W. FOSTER-HARRIS Look of Old West viii. 234 Prairie or mountain oysters were an unmentionable part of a male animal.
1992 Financial Post (Toronto) (Nexis) 12 Dec. S15 The restaurant’s signature dish, Prairie oysters, deep fried with salsa.