The various forms It takes

Are there any theory on the various forms It takes?

I have a theory that they are the forms of previous people it has killed, since after it kills Greg it takes his form to follow after Jay.

Just before Greg dies It is banging on his door and he yells "what the f*ck mom". Some might think it took the form of his mom who is alive, but that is not the case. He merely thought it was his mom banging on his door, since when he opened it was surprised by It tackling him to the ground.

Edit:
I also just realized how messed up it makes that scene where It turns into a little girl when it was attacking Jay and her friends at the beach. Just think about that for a second. If theory this is correct then that girl was probably raped and thereafter killed by It.