I mean, you should get around to just watching the show and comics/books. Lots and lots of characters to work off of.

Queen Chrysalis being a bug monster easy a super easy creepy art piece you can do.

Celestia and Luna have alter egos being Nightmare Moon and Daybreaker. Nightmare Rarity was a thing too, in the comics. Princess Amore is a non-alicorn royalty that was transformed to crystal and shattered in the comics, but is hinted at not being truly dead even so.

Other creepy things you can work off of is "swamp fever" that kills ponies turning them into trees.

Pony of Shadows that uses ponies as hosts and "leaks" out of there eyes and mouth.

Tree of harmony draining the life out of improper users of the Elements leaving them as husks.