who is "IT ''? why does Melinda to refer to this person as that ?

It is Andy Evans, Melinda's rapist. She calls him IT or Andy Beast in order to separate herself from who he is and what he did.

I see FT in the hallway. FT goes to Merryweather. IT is walking with Aubrey Cheerleader. IT is my nightmare and I can't wake up. IT sees me. IT smiles and winks. Good thing my lips are stitched together or I'd throw up.