Cut hot dogs to create knuckles, by first making a shallow straight cut. Then make another shallow cut at an angle to the first cut and remove the small wedge of hot dog. Repeat.
To make the nail beds, cut two shallow 1/2 inch cuts at the end of the hot dog about 1/2 inch apart and cut across the two cuts to mark the nail bed. Using a pairing knife, carefully carve out the cut section of the nail bed. Repeat.
Cut an "X" in the non-fingernail end so when cooked it will create a flared jagged end of the finger. (Optional: Add a gruesome bone to the jagged end by sticking a small piece of raw potato or small piece of string cheese into the jagged end of the hot dog and cover with ketchup blood.)
Cut red onion in half and peel off a large outer ring of onion. Slice outer ring of onion lengthwise with the grain of the onion into 1/2 inch wide fingernails and trim the length as desired. Cut 8 and set aside.
Test fit of fingernails. It's okay if they are a little wider than your nail beds, because the hotdog will puff up a bit when cooked.
Boil hot dogs according to package directions. The hot dogs will puff up a bit and that gives the fingers a realistic look.
Add lots of ketchup to each bun and place hot dog fingers in buns. Dip the fingernail onion into a little ketchup to act as glue to hold the nail on and add it to the hot dog nail bed. Squeeze more ketchup and serve!