Infante Felipe of Spain, duke of Calabria, elder son of Carlos III, was excluded from the succession.
Not sure if he was "forced to renounce" as I don't think he had the capacity to understand.
Ludwig II of Baden was notorious mentally ill (comes with the name
) and his brother Frederick acted as regent on his behalf.
However, two years before he died, he was deposed and his brother took over as Grand-Duke.
King D.Afonso VI of Portugal was deposed by his brother D.Pedro but opposite to the Bavarian case, he acted as regent as long as D.Afonso was alive, and only became king after his death.
Same for D.Maria I and her son D.Jo„o VI
Who were some notable European royals who were in line to inherit thrones, or even succeeded -- only to either be deposed or forced to renounce their succession rights because of mental incapacity?
Other than the famous mad kings of Bavaria (i.e. Ludwig II and his brother, Otto, who nominally succeeded him), some examples coming to mind would be Czar Ivan V of Russia (the elder half-brother of Peter the Great), Prince George of Serbia (eldest son of King Peter I Karageorgevich, older brother of King Alexander I), and Emperor Ferdinand I of Austria.
Juana la Loca of Castile might count, since she was allowed to briefly rule the kingdom she inherited from her mother, Queen Isabel I. But between them, her father, husband, and son eventually took over. And, of course, she was not allowed to inherit -- much less rule -- her father's kingdom of Aragon. Instead (I believe), she got bypassed in the succession in favor of her son, Holy Roman Emperor Charles V.
I'm sure there are many more cases. Can anybody name them?