In many works of fiction, the ruler of a country or piece of land will often share their surname with the land itself. There can be a few reasons -- the country may have been named by the ruler after themselves, or the ruler may be an outsider who took the country's name as their own to fit in. It may also simply be tradition for the current ruler to take the name of the country as a surname when they come to power.
This practice comes from the old naming conventioin to refer to nobility as "<name> of <birthplace>", such as Henry of York. This was known as a "territorial surname" and was used to specifically note that the person was a landowner or noble of some sort. In fiction this can occur due to authors not having knowledge of noble houses, or the proper history behind territorial surnames. In short, it's simply easy to name the ruler of a country after the country, it saves the writer getting into the politics of the political houses and it's easier to remember for the reader.
- Code Geass, all Britannian royalty has the last name "Britannia" with some prefix, like "di Britannia" or "li Britannia".
- In Dragon Ball Z, Vegeta is named after his planet
- The Wicker Man: Both the 1973 original, and 2006 remake have these; the former being Lord Summersisle, the latter being Sister Summersisle.
- In The Queen's Thief, the monarchs of the three countries (Sounis, Eddis, Attolia) are referred to by the country they rule. Although they all do have individual names they were born with.
- The Land of Oz is at first ruled by the Wizard, who calls himself Oz. We later learn that the land was already called Oz before he got there and the fact that the Wizard was also named Oz was a coincidence. The Wizard thought they had named the country after him. From the 2nd book onward, Oz is ruled by a princess named Ozma.
- The Forgotten Realms setting flipflops on this with regards to the city-state of Neverwinter. According to some sources it's called that because its river never freezes (due to being heated by a nearby volcano). Other sources point to it being a truncation of "Never's Winter" after Lord Halueth Never, its earliest ruler.
- In Traveller the head of an Aslan clan is referred to by the name of his clan. When distinguished from his clan as an individual the syllable "ko"("chief" or literally "himself") is added. That is the head of Coolclan is formally either Coolclan or Coolclanko.
- Final Fantasy V, Tycoon is ruled by Alexander Highwind Tycoon and Karnak is ruled by Queen Karnak.
- Final Fantasy VI, Edger Roni Figaro rules the kingdom of Figaro.
- Final Fantasy IX, the royal family of Alexandria bear the name "Alexandros". Both double as foreshadowing of the kingdom's guardian Eidolon, Alexander.
- Final Fantasy XII, Ashe's last name is Dalmasca. Subverted with the Archadian emperors, who are of House Solidor and use that as their surnames, never using Archadia or any permutation of the name.
- Ultima, Lord British rules over Britannia. In this case the land was named after him, he united the land, then called Sosaria, under one banner and rechristened it Britannia.
- The Legend of Zelda, a couple games give the royal family of Hyrule the last name Hyrule. Both are justified, as the land was once ruled and protected by the goddess Hylia, and Skyward Sword reveals the first Zelda was Hylia's reincarnation and is implied to have resettled Hyrule with humans after the game's end.
- The monarchs of Guardia in Chrono Trigger in 600 AD and 1000 AD are known only as King Guardia XXI and King Guardia XXXIII respectively.
- The Empire in The Elder Scrolls is sometimes called the Septim Empire after the surname of its royal family through Oblivion. This is mainly to distinguish it from two previous Empires in the setting.
- Commander Canada and Commander Canadia from the Keegan's Truck web series.
- Saudi Arabia, ruled by the Saud dynasty
- Liechtenstein, ruled by Princes Liechtenstein.
- Though he formally has no surname, the British Prince William, in his military career, uses the surname Wales because he is the Prince of Wales. The same goes for the rest of his family. His brother is known as Captain Harry Wales.