Firstly, Uoht is apparently one of the members of the Imperial family mentioned in Chambers' The Repairer of Reputations: Mr. Wilde "...led Vance along the ramifications of the Imperial family, to Uoht and Thale, from Naotalba and Phantom of Truth, to Aldones...".
In the texts of James Blish and Lin Carter, Uoht is the son of the queen Cassilda, as in The King In Yellow by Thom Ryng. It is normally considered that he and his brother Thale are rivals in love for the hand of their sister Camilla.
In other sources the name is attributed to a king who wore the Sign of Zoth upon his cloak. It seems likely that one takes their name from the other.
The Encyclopedia Cthulhiana says...Edit
Under his own name, the Encyclopedia Cthulhiana states two alternatives. He may simply be a "royal contender for the throne of Yhtill in the play 'The King in Yellow'." However, he is also "according to Hali, the first king of Carcosa to emblazon the Sign of Zoth in yellow upon his cloak."
Under the entry for the play, two alternatives are suggested. Either he is simply one of several claimants to the throne of Yhtill or he is one of two sons of the ruling queen of the city of Hastur, which has been at war with its neighbor Alar for countless years.