Lost Orphaned Royalty

Lost Orphaned Royalty is when a child, raised as an orphan, turns out to actually be the next heir to the throne, or similarly destined for a position of authority or political power. There are a multitude of plots that could stem from this trope: The parents could have sent him away so he could have a "normal" childhood, the orphan could be well aware of his royal status, but just needs a way to get it back. The orphan could suddenly be thrust into the big chair that is the monarch. So there are a lot of possibilities when it comes to orphan monarchs.

If the villain is all like, "I will kill the king and his descendants so I can take the throne", then the orphan can be on the run from the villain trying to kill him... Or he could be the overlooked heir that the villain didn't realize was a rightful heir to the throne. The orphan could be the villain, obsessed with getting the life he deserves to have, being the true heir and all...

Suddenly-Suitable Suitor is a common plot twist.

Compare Moses in the Bulrushes, Changeling Fantasy and Switched At Birth.

WARNING! There are unmarked Spoilers ahead. Beware. Some plots may be utterly spoiled by knowing who's the real royal too early.

Examples of Lost Orphaned Royalty include:

Anime and Manga

Mai-Hime: Arika Yumemiya. Maybe. The series goes out of its way to muddy the waters, and she personally doesn't seem to care. All we can be really sure of is that Mashiro, the girl currently held to be the princess, isn't.

Film

Lone Starr of Spaceballs

Literature

Shasta, from C. S. Lewis's The Horse and His Boy is one.
Subverted twice in Discworld, with Tomjon in Wyrd Sisters and Carrot in the various Watch books both not wanting the job.

Theatre

In Stuart Patterson's play Cinderella, Cinders rejects Prince Charmless in favour of Callum the stable boy. Who then turns out to be Lost Orphaned Royalty of a neighbouring kingdom.