From ErfWiki

Proposed Canon

"Prince" and "Princess" are Royal titles for the popped heirs of Kings and Queens.

Known Princes and Princesses are:

• Prince Ansom (Former; now claims no royal title)
• Jillian Zamussels (Former; now Queen)
• Don King's son and daugher (Former; both croaked)
• Prince Tramennins, son of King Slately, brother of Ansom
• Prince Ossomer, also son of King Slately and brother of Ansom

Speculation

"Prince" and "Princess" could be honorifics for royal units popped as heir to a side ruled by a King or Queen.