Mr. Robert "Bob" Gray
Think of it like this: Bob Gray was possibly a real person, and he, in fact, was possibly Pennywise the clown (as a job/living) in real life, at some point in the past in Derry.
(I also want you to understand that Mrs. Kersh is IT, not some offspring of IT.)
The backstory of Robert Gray... which does not exist in Stephen King's literature... may be a creative liberty taken by Andy Muschietti in It Chp. 2.
Since the 80’s, readers have speculated that Robert “Bob” Gray was just a regular man who made a living as a clown. The monster/creature, known as IT, possibly drove Gray mad, killed him, and shapeshifted into his likeness as the clown. ITs motivation being that doing such would make hunting and killing easier.
In other words: IT, the monster, (who is billions of years old) killed Bob Gray and essentially "stole" that persona via shapeshifting, because that made things easier for hunting. (To explain: IT will hunt and eat any creature. But IT prefers to eat creatures that get scared the most because fear "salts the meat." Ergo, IT hunts people the most. IT discovered that human children are the easiest to hunt and scare. IT then realizes that posing as a clown makes this all the more easy...)
It's like "Pennywise" becomes this familiar name to people, but can't be real because it was over 100 years ago that the actual man had died.
Here are photos from my actual copy of IT, revealing the exchange between Mrs. Kersh (IT) and Beverly about Kersh’s “fadder” named Robert Gray.