A prairie oyster is a bull testicle (not COW testicle, since FEMALES DO NOT HAVE TESTICLES!). It's a delicacy of the western and
mid-western, semi-arid grasslands of North
America (basically everywhere cattle are bought and sold).
To my understanding, they are quite good when double-battered and deep-fried in
butter, generally mixed with sauteed vegetables or whatever else you heart desires. Being from the prairies myself, I find that they are quite tasty when eaten with sauteed mushrooms and beef gravy.