In movies and tv-series it often seems to be a gay prince whom is heir to the throne (Braveheart, GoT, etc.) But are there any real instances of this happening, and if so was it common or is it just hollywood being hollywood?
I'm not going to type the entire reply again, but I've previously covered Chinese kings and emperors (especially during the Han dynasty) who were at least bisexual, if not homosexual. So yes, there were historical heirs who did prefer company of their own gender.
The short answer is yes, history is peppered with royalty who preferred their own sex.
Historian John Boswell has written extensively about homosexuality in the Middle Ages, and published 3 great books on the subject that you should look into if you're interested in the topic.
Christianity, Social Tolerance, and Homosexuality: Gay People in Western Europe from the Beginning of the Christian Era to the Fourteenth Century, Same-Sex Unions in Premodern Europe and Rediscovering Gay History: Archetypes of Gay Love in Christian History.