I think I'll take this in order of scariest illustrations.
"The roof space is creeping and crawling with things, things that have horns and raggedy wings."
(In the Haunted House, by Eve Bunting and illustrated by Susan Meddaugh)
The first read of this is scary with two sets of feet walking through a house full of scary creatures. Knowing the twist at the end - a little girl drags daddy through this pretend Halloween house again - helps kids appreciate following reads.
""Oh," said Dave. "Why is it...trying to eat my burger?""
(Are the Dinosaurs Dead, Dad?, by Julie Middleton and illustrated by Russell Ayto)
These dinosaurs are pretty cute. Well until the end. Their teeth are a bit sharp. And their proximity a little close. And their sneakiness is a bit scary.
(The Gruffalo, by Julia Donaldson and illustrated by Alex Scheffler)
I dunno, the Gruffalo is kinda cute. But I have to admit I fell into a false sense of security that the Gruffalo wasn't real. Well, the little mouse pulled one over on me... BAM! Oh, no! A Gruffalo!
"Those spooky, empty pants and I were standing face to face!"
(What Was I Scared of, by Dr. Seuss)
Now, I don't know if green pants with nobody inside them are creatures, per se, but they certainly are something creepy. Super creepy. Because an apparition who can put on pants could do any number of scary things. And why green? To hide behind Brickel bushes and scare us? Yes! The ending is supposed to give us closure - aww, look they're friendly! - but all I keep thinking is why are those green pants talking out of their butts?! Scary.
"A ghoul! A ghastly drooling graveyard ghoul..."
(By the Light of the Halloween Moon, by Caroline Stutson and illustrated by Kevin Hawkes)
Combination of scary creatures and a scary setting and a little girl about to lose a toe, collude to make this near-horrifying. The witch sports uber pointy nose, matching chin, gnarly teeth, and pop out of her ead wide eyeballs. A pirate ghost looks real. (if you've ever seen a pirate ghost, you'll know what I mean.) A sprite has no cover of cuteness, he's just creepy. Luckily this little girl is tough as nails and gives all these ghastly creatures a swift whack to save her toe.
" ...so the goblins pulled baby out, leaving another all made of ice."
(Outside Over There, by Maurice Sendak)
Classic dystopian picture book by the master. The goblins hidden in their robes are scary, but they turn out to really just look like babies. Thank goodness for that relief, but it doesn't fix the problem of what my imagination had come up with. The scariest creature in the book is actually the baby made out of ice the goblins leave when they steal Ida's baby sister.
(Cheese Belongs to You! By Alexis Deacon, illustrated by Viviane Schwarz)
I learned a lot about rat law in this book, which unfortunately is kinda close to human rules. The rats in this book get progressively scarier, culminating in a giant scary rat brawl. I always find one critter may be not too scary or even kind of cute, but a whole mess of critters always creeps me out.
"His horns were bright red and his cape midnight black, and his pencilly fingers tapped "clickety-clack.""
(The Monster Who Did My Math, by Danny Schnitzlein, illustrated by Bill Mayer)
Totally cute idea of a monster who does your math, but makes you look like a fool when everyone finds out you haven't learned anything. Cute except the illustrations are incredibly horrifying. Like scary clown jumping out of a barrel of diseased monkeys in a haunted house horrifying.
I'm sure you've read some picture books with SCARY creatures, so please share them in the comments. I have to say it's surprising just how scary some of these creatures can get!