Has real Robert Gray actually existed?

Question

At least in two occasions, the It creature introduces itself as a "Robert Gray, better known as Pennywise the dancing clown". First Mrs Kersh tells about him:

"My father [...] His name was Robert Gray, better know as Bob Gray, better known as Pennywise the Dancing Clown. Although that was not his real name either. But he did love his joke, my fadder [sic]"

Then Henry Bowers receives a switch-knife in a package sent by "Robert Gray".

Has the person called Robert Gray actually existed? Has King used this name in any other stories?

Answer 1

The answers to all your questions is really "we don't know".

Has the person called Robert Gray actually existed?

It's hard to say. As you say: It, in the form of Mrs Kersh, does claim to be the daughter of Robert Gray aka Pennywise the Dancing Clown aka It itself. We never learn whether or not either Robert Gray or Mrs Kersh are or were actual persons that It impersonated.

There are theories online (1, 2) that Robert Gray was an actual Derry citizen who played a clown (King was probably partly inspired by the real-life serial killer John Wayne Gacy aka. Pogo the Clown), and that It might have killed him a century or more ago and assumed his persona ever since. Those are just theories, however. If they're true, that person might very well had a daughter who grew up to be Mrs Kersh.

Of course, It was actually a female (insofar as It's alien race had genders), who laid eggs and had actual offspring as well.

Has King used this name in any other stories?

In the novel Dreamcatcher, which takes place in Derry, Maine just like It, there's an alien entity calling itself "Mr. Gray". Whether this entity has anything to do with It is not clear. The book makes several references to It, including a sighting of a clown in a storm drain and the graffiti "Pennywise lives!" (perhaps implying that It did not die in It, or that one of It's offspring might have lived and assumed the Pennywise persona?), and it's said that the murders there were committed by a psychopath dressing up as a clown.

All of this is conjecture, however, but we don't have any Word of God to go on here.

Answer 2

In It there is mention of a bartender named Jonesy in Derry back in 1905, working behind the bar the axe murder occurred. And the alien Mr Grey in Dreamcatcher is inside “Jonesy’s’ head throughout part of the story, so there are more name connections.

Answer 3

In the short story The Library Policeman, the character of Ardelia Lortz is revealed to be a creature much like Pennywise and is even called "it" but in lowercase form, not It. She has eyes "like silver nickels" and feeds on children's fear. It is mentioned that whatever it is, the main characters believe that Ardelia Lortz was once a real person that was hollowed out "like a pumpkin" so that it could inhabit her form. She wants to do the same to the story's protagonist, Sam Peebles. So I think it's possible that Robert Gray did once exist.