Boo!

The Great Old Pumpkin
The Insects from Shaggy

That second one isn't actually all that good, but I thought the gang ought to be represented in some way. And let me interject here that I am rather disappointed in the "fanfic" community, with regard to Lovecraftian crossover stories. The only Lovecraft-Scooby crossover that seems to be available is basically just a big dirty joke, and I can't locate a single Lovecraft-Nancy Drew crossover story at all.