The Big Eye dog starts transforming into a hideous monster.
I'm not a real big fan of this sequence, I'd do it differently
if I had it to do again. The rough "evil energy" effect around
the dog was achieved by dry-brushing with India ink.
The dog name-drops "Pazuzu" -- in the godawful sequel Exorcist II:
The Heretic, it was decided upon at some point that "Pazuzu" was the
name of the demon who possessed Reagan in the first movie. (And here
I thought it was supposed to be Satan himself! What a let-down.)
Anyway, there's some horror trivia for you! You should see the movie
just to see James Earl Jones dress up like a witch doctor to
battle demons.
You folks who remember Dungeons and Dragons from when you
were a kid might also recall Pazuzu from the pages of the much-loved
(for the pictures) second D&D monster book, Fiend Folio.