King Juan Carlos is rumoured to have several illegitimate children, like his grand-father and great-grand-father Alfonso XIII and Alfonso XII.
Interestingly, his father, the late CoB, was a notorious womanizer but never perspire the existance of any child.
Prince Amedeo Duke of Aosta has an illegitimate daughter Ginevra with Kyara Ellynkhuizen.
Prince Michel de Bourbon-Parme had an illegitimate daughter Amélie with Laure Le Bourgeois.
He was also the father of Pss Maria Pia's 2nd pair of twins but they were recognized by Maria Pia's husband so they cannot be considered illegitimate.
Pr.Michel's daughter, Pss. Ines had an illegitimate daughter Mélodie de B.Parme, later adopted by grand-father Pr.Michel.
Pr.Michel had another son, Pr.Eric, whose daughter Pss Gabrielle had an illegitimate son, Pr.Mark de Bourbon-Parme
Prince Carlos de Bourbon-Parma had a son Hugo with Brigitte Klynstra.
Prince Nicholas of Romania was stripped of his royal apanage by his grand-father King Mihail due to the suspition - later confirmed - that he might have fathered a child with a woman he was not married with.
Prince Heinrich of Hannover has an illegitimate son Oscar with Désirée Nick