The testicles of young calves. My SO (who has actually eaten them on several occasions) says that they are best when fried in oatmeal and they're fresh. The calf testicles are apparently much better (less stringy or tough or something) than bull testicles.
We have a friend whose parents own a large ranch. They invited some friends out for their branding, and they brought their son, who is slightly odd (we think that he is perhaps slightly autistic). Anyways, he REALLY loved the prairie oysters. He kept asking,
"May I have another testicle, please?"