It's a huge misconception known only by people who have read the novel or know things that the monster's name is Frankenstein. In "reality," it has no name, and for good reason, which you'd know if you've read it.
In the novel, Frankenstein's Monster is referred to as "the creation." The book is named for its narrator/protagonist Victor Frankenstein, the creator of the being.
He didn't grunt and speak like an infant, either. He was quite eloquent.
