It's inevitable for details to change when translating a book to the big screen. Directors and their respective distributors have to target their audience, and that sometimes means toning down certain elements of a narrative in order to reach more viewers.
New concept art from Jody Revenson's book Harry Potter: The Creature Vault reveals that the Hollywood franchise could've been a lot darker than what was released. Granted, the films' ratings increased from PG to PG-13 when the fourth film rolled around (except for The Half-Blood Prince, which oddly received a PG rating), but those ratings would hardly allow for some of these creatures to come to fruition.
The Dementors are more Grim Reaper-like than the diaphanous beings audiences meet in the third installment. Fenrir Greyback, the wizarding world's most savage werewolf, looks more like a demon wolf than the brute with sharp teeth we see in the films. And, Nagini? Nope. Nope. And, more nope.