There's at least a few characters they didn't create. Baba Yaga and Iblis come from ancient folklore. Uhura was a character in Star Trek. Are there any other ones?

There's all of the comedian cameos that should go without saying.

Suleiman ben Daoud is literally Solomon, Son of David, just as a wizard-king instead of a wise king.

Harun Al-Rashid was a real person, and his vizier/interpreter, Ja'afar, is likely based off of at least one of a number of viziers who were named Ja'far - which Aladdin's Jafar was also named after. The idea of a Sultan disguising himself as a poet (or was it a commoner?) and wandering among the townsfolk is also attributed to a historical individual IIRC, but it wasn't Harun himself. I forget who it was, at the moment.

There are probably a lot more mythological and historical references in Quest for Glory 2 that I can't quite think of at the moment, but that game in particular is worth looking into.

Quest for Glory 3's Sekhmet is, of course, taken from Egyptian mythology.

Igor from Quest for Glory 4 originates from the Frankenstein novel, which is often considered the first science fiction novel in history.

Titania in QFG4 is based off of the Titania from Shakespeare's A Midsummer Night's Dream.

Minos in QFG5 might also count, although he doesn't appear to have much in common with his historical namesake and it's probably just there to hint at his royal ambitions and further hammer home the "obviously villain" neon sign floating above his head.

Does Mort being based off of Frankenstein's Monster count?

Alas, poor Yorick!

Hard to say, because Mort is more based on the pop culture surrounding Frankenstein's monster than the original source material, and those two concepts have diverged fairly substantially.

I don't remember a Titania in QFG4.
I forgot about Ferrari and Ugarte. Those characters seem to be a cross between their namesake characters in Casablanca, and some character in the Maltese Falcon. I thought his name was Gort with a "G."

The scientist who created him could be based on Dr. Jekyll and Mr. Hyde, although maybe not completely because the original character was one good personality and one evil personality. Dr. Pretorious and Dr. Mobius are both evil, or at least they're evil to wizards and magic users. There doesn't seem to be much difference in their personalities other than their favorite pizza.

Isn't she the Queen of Fairies? Or was her name something else?

She is exclusive to the Wizard playthrough. She does not appear if you are playing as any other class, even if you have magic ability. Sounds right to me. To be fair, he's a pretty forgettable character. The only memorable thing about him is what he says if he decides to sacrifice himself to the dragon at the end of the game.

Pretorius and Mobius seem to be generic mad scientists. Their dual personality may well be a Jekyll and Hyde type deal, but they take inspiration from other sources as well so I think they're their own unique characters who just take inspiration from other examples. Yes, Titania is the Queen of the Faeries (faeries and fairies are two different things in the QFG universe) and a character created by William Shakespeare for A Midsummer Night's Dream. She has such a short appearance in the QFG series that there is relatively little to her character beyond just that, "Titania, Queen of the Faeries"

That fairy in QFG4 is named Tatiana, not Titania. I suppose she could've been based on Titania, never realized that before, but her QFG4 name is a little different.

Wizard-kings and wise kings aren't mutually exclusive, and the idea that Solomon commanded djinn comes from Islam (if it didn't already exist in Judaism).

Erasmus was the name of a Dutch monk and scholar from the Renaissance era.

Tatiana, the name of the Faerie Queen, is a Russian name that just happens to be almost an anagram of Titania.

The Rusalka and Leshy are both taken from Slavic mythology. Cerberus is direct from Greek mythology. The Minotaur (Toro) and Dryad are both from Greek mythology, although they both first appear in the first game which largely contains creatures from Germanic mythology. The Kobold is a creature from Germanic mythology. Arne Saknoosen is a play on Arne saknussemm the cryptic alchemist from A Journey to the Center of the Earth.

One of the many nods to Monty Python, the guardian of Erasmus' home.

Ah yes, also the killer rabbits in QFG4.

An Forda and Son, pretty much the titular characters of the TV series Sanford and Son.

Dr. Cranium, a crazed relative(?) of Dr. Brain from older Sierra games - The Castle/Island of Dr. Brain.