From ErfWiki

A King is a male Ruler that is a Royal, while a Queen is a female Royal ruler.

Proposed Canon

Known Kings include:

• King Saline IV of Gobwin Knob, (croaked)
• King Slately of Jetstone
• Don King of Transylvito
• King Dickie of Haggar
• King Banhammer of Faq, (croaked)
• King County of Transylvito, (croaked)

Known Queens include:

• Queen Jillian of Faq
• Queen Bea of Unaroyal, croaked

Speculation

Only Units who were popped as an Heir can become a King or Queen. Any unit who was promoted to Heir Designate - even a noble - becomes an Overlord when their Ruler croaks, rather than a King or Queen.

This may mean that previously Royal sides can lose the ability to pop royal units/heirs if an Heir Designate rises to the throne.