Many Portuguese Kings were not the first-born son:
First king, D.Afonso Henriques , was the second surviving son of Henri de Bourgogne and Teresa of Castille.
His elder son Infante D.Henrique died in infancy leaving place to King D.Sancho I
D.Sancho's elder son D.Raimundo died in infancy so he was succeeded by D.Afonso II
Two of D.Sancho 's sons seek fortune abroad.
Infante D.Pedro became Count of Urgel and King of Majorca (Consorte).
Infante D.Fernando became Count-Consorte of Flanders
Back to D.Afonso II,or the first time a first-born succeded his father, D.Sancho II , but years later he was overthroned by his younger brother D.Afonso III .
D.Afonso III was succeeded by his 2nd son D.Diniz , and was succeeded by his only son D.Afonso IV , who was succeeded by his 3rd son D.Pedro I .
D.Pedro was succeeded by 2ns son D.Fernando I , who, after the death of his 2 sons, had one daughter D.Beatriz , sadly married to the king of Castille.
Although proclaimed as Queen of Portugal, her queenship was never recognized.
Followed D.Fernando's half-brother D.João I , succeeded by 2nd son D.Duarte I .
D.Duarte's 2nd son became king D.Afonso II and was succeeded by 2nd son D.João II .
D.João II's only surviving son died from a horse fall so he was succeeded by his cousin and brother-in-law D.Manuel , 10th child and 6th son of Infante D.Fernando, Duke of Beja and Viseu.
D.Manuel's firstborn was little Infante D.Miguel da Paz who would reign over Portugal, Castille and Aragon but saddly died in infancy so his 1st son from marriage #2, D.João III , succeeded to the throne.
All of D.João III's 9 children died before the king so he was succeeded by his grandson D.Sebastião who died in battle in Morocco, unmarried and w/o children.
The throne was inherited by D.Manuel's 6th son Cardinal D.Henrique I after whose death, Infante D.António proclaimed himself king.
D.António was the natural son of D.Manuel's 3rd son Infante D.Luis.
Then came the spanish Habsburgs Felipe I (II) of Spain, II (III of Spain) and III (IV of Spain)
And then came the Braganças with D.João IV , elder son of Duke D.Teodósio.
With "the curse of the Braganças", issued by a revengeful nonk, no surprise that no first-born ever succeeded to the throne, but that is anothe story, that I will continue later as I must leave now.
What are notable instances in royal history of youngest sons -- other than only sons -- eventually becoming reigning monarchs? This is a somewhat broad question, and I'm willing to accept a variety of answers.
A prince could theoretically have been born the last of (say) five sons in his family, but still succeeded to the throne because his older brothers all died in infancy. But he would still count as a youngest son who became a reigning monarch.
King Alfonso XIII of Spain, however, would not -- for reasons that he was not preceded in birth by a brother who died in infancy. Holy Roman Emperor Ferdinand I (founder of the Austrian Hapsburgs), however, does count -- despite having only one older brother (Holy Roman Emperor Charles V, founder of the Spanish Habsburgs).
Later on in the history of the Habsburgs, Archduke Ferdinand Karl (youngest son of Holy Roman Emperor Francis I) became the Duke of Bresgau and founder of the cadet branch known as Austria-Este.
Prince Valdemar of Denmark (youngest son of King Christian IX) would have made the list, but for the fact that he refused the throne of Bulgaria which got offered to him.
King John of England, however, counts -- despite having usurped the throne. There was no clear succession law as of his day, where basically possession was nine-tenths of the law. Similarly, King Henry I also counts, for reasons that he was the youngest son of William of Normandy ("the Conqueror").
Can anybody name additional examples?