Roman emperor Claudius perhaps. Last man standing after all the other male relatives of the imperial family had perished. Considered a disgrace and an imbecile and found hiding behind a curtain when dragged forth by the common troops of the imperial guard to be proclaimed emperor.
Well there are close examples, but not really the exact same example as Aegon the Unlikely.
King Henri III of France ( rule 1574-1589 ) was the fourth son of Henri II. Two of his older brothers were kings before him :
François II ( rule 1559-1560 ) who married Mary Queen of Scots but died childless aged 16.
Charles IX ( rule 1560-1574 ) famous for his terrible health, his implication in the St. Bartholomew's Day massacre and living under his mother's hand.
And there was Prince Louis, who died as a child but was still older than Henri III, making him a fourth son. Note that Henri III also died childless thus ending the Valois line in France and opening the way for the Bourbons ( Henri of Navarre, future Henri IV became king after him ).
Also, Henri III's, François II's and Charles IX's father, Henri II was a second son himself ( his brother had died before their father ) and their grandfather, François I was a distant cousin of the previous King ( Louis XII ) and became heir only because Louis did not have any surviving sons.
Still in France, Louis XIV lost almost all his direct heirs before passing away himself. His son, * the Grand Dauphin* died in 1711. His grandson the Dauphin ( the Grand Dauphin's son ) died barely a year later in 1712. The Dauphin had lost two sons at a young age, the Duke of Brittany and the Duke of Burgundy. The third son finally inherited his great-grandfather in 1715 and became Louis XV.