One testicle not descending into the scrotum during normal embryonic or fetal development (3–4% of 'normal' live births), also known as undescended testis or cryptorchidism. In this case the testis is within the abdominal cavity, somewhere along the normal route of descent – most commonly, within the inguinal canal. Such a testis has an increased risk of malignancy.
Archibald Douglas, 4th Earl of Douglas, magnate of the Kingdom of Scotland, and Peer of France. Lost in 1403, while fighting at the Battle of Shrewsbury (The previous year he had lost an eye at the Battle of Homildon Hill).
Adolf Hitler (Allegedly), Sources from historians vary on the topic of Hitler's monorchism. Sources vary from Hitler being shot in the groin by a French Soldier during the battle of the somme to it being a birth defect. The truth as to why Hitler may or may not have only had a singular testicle, will be unlikely known, due to the fact that he was cremated after his apparent suicide.
Monorchism in nonhuman animals
Although extremely rare, monorchism has been observed to be characteristic of some animal species, notably in beetles.
An individual having monorchism can be referred to as monorchid.