Why do some people, especially royalty, get more than one name? One that sticks in my mind is Ramses also being called Ozymandias (I think it was the Greeks who called him that?), but I’m sure there are many more examples. What, couldn’t the Greeks have just called him ‘Ramses’ or couldn’t they pronounce it?
Well, with Ozymandias/Ramesses II, there are a couple things going on.
Ramesses is the pharaoh’s personal name. His throne name was “User-Maat-Re-setep-en-Re”, the first part of which the 1st century BC Greek historian Diodorus Siculus rendered in Greek as Ozymandias. How exactly he got from A to B I have no idea, as they don’t sound too much alike to me, but we are dealing with three languages, two of which are rendered in different alphabets and the third in hieroglyphs. Oh, and vast stretches of time in which phonologies could have shifted around. You probably noticed that Ramesses has a few different transliterations, for example.
For extra credit, User-Maat-Re-setep-en-Re means “Strong is the Truth of Re, chosen of Re,” Re being the sun god.
Pharaohs actually had five names apiece. Why? Well, each had a specific function–the birth name was usually a family name, the throne name was taken at the time of accession and was used for official business, and the others seem to have had religio-political functions. I think that other cultures have used “throne names,” but I don’t know any specifics about that–I’m sure somebody else here will chime in shortly.
I think it is because they translate the names from one language to the other. Take Charlemange. I think that is old French for “Charles the Great”. Now imagine that Charlemange was Egyptian and the Greeks spoke English. History books would then say something like “Charlemagne, the pharoah (or ‘Great Chucky’ as he was known to the Greeks) was a god-on-earth…”
Therefore, Charlemagne == Greatchucky
Sometimes they would translate the name (use the Greek word for ‘bird’ if the Egyptian word was used) or they’d just transliterate the sound (Egyptian probably had a glottal ‘q’ sound that ancient Greek didn’t, so ‘q’ became ‘k’, etc).
Another thing is that, in some places, the new king picks a new name when he ascends to the throne. The most recent person I can think of to do that is the pope, who was born Karol Wojtyla and is now called John Paul II.
You also have kings getting odd nicknames attached to them, like Pepin the Short, or my personal favorite, Constatine V Copronymous (“Constantine Dung-name”, so called because he soiled the baptismal font during his christening). You have some kings surnamed “Barbarossa” or “Ahenobarbus”, both of which mean red-bearded.
Mistranslations help a lot too. Wasn’t there an early British king named Ethelred Redeless (“without counsel”) who’s better known as Ethelred the Unready?
**I’d like “John” for my throne name, please. **
Of course, your Majesty. And here’s your copy of “Uncle John’s Bathroom Reader” to go with your throne 
I heard that Ethelred was first called “the Unready” by some historian about 500 years after he died. At any rate the pun seems pretty obvious:
Ethered
Ethered the Un-rede (without counsel)
Ethered the Unready