This has plagued me for several years. Who is the most likely individual responsible for bestowing Xerxes' name upon him. Himself? His father Darius? Did he have a birth name and then chose Xerxes as a regal name?
I ask as a minor point relating to the notion that in Aeschylus' Persians, Darius insists to Xerxes that he not invade Greece, that war is foolish. This is almost certainly a fabrication, but I would like to know if there is evidence of Darius naming Xerxes, which I understand means 'Warrior' in their tongue.
"Xerxes" is written in Achaemenid Persian as "Khshayarsha", and has two components: "Khshaya-", and "arsha". "Khshaya" is Persian for "royal power", cf "Khshayathiya", which means "King" and is the antecedent of the modern Persian "Shah". "Arsha" means "Man" or perhaps "Warrior". The proper translation of Xerxes would probably be something like "Ruler of Men" or "Ruler of Warriors", the closest equivalent Greek name I can think of would be Anaximandros, from anax ("king") and "andros" (man).
(Edits below for clarity, I mixed up a few trains of thought when writing this the first time)
Like the similar name Artakhshaca (Artaxerxes - Khshaca is a dual of Khshaya, probably from Median, meaning "Righteous rulership"), it would be conceivable prima facie that it was a throne name, although this cannot be the case as the name appears in Persepolis fortification tablets that precede his rule (see /u/Trevor_Culley below). We don't know who would have named Xerxes if this were not his birthname, but he was a favourite at least in part owing to his influential mother, Atosa, being one of two remaning descendants of Cyrus, and also in part due to being "born in the purple", while his father had ascended to the throne. He supposedly faced a challenge by an Artabaza, the eldest son of Dareios, but he was not able to seriously challenge Xerxes, and it is probable by considering the longevity of many Persian kings that primogeniture was not the norm. There isn't that much commentary that I know of relating to the naming of any kings - Artaxerxes I and II have the names Arshaka noted in some Babylonian contexts, and Dareios III was named Artashata prior to his ascension to the throne.
As far as I know, no sources note any alternate names for Xerxes. It's possible that Xerxes was so named at his birth, it is possible that he was given the name as an adult, it's possible he took the name on his ascension (see /u/Trevor_Culley below!) - but I would guess that he at least had the name Xerxes before becoming king, otherwise I'd almost expect some source to note alternate names for him.