Cut the eggs in half lengthwise. Remove the yolks and place them in a small bowl.
Mash the yolks with a fork until fine crumbs form.
Stir in the mayonnaise, butter, mustard, salt, and pepper until smooth. Spoon the yolk mixture back into the whites.
Cut the olives in half. Place one-half of an olive on each egg to form the body of a spider.
Thinly slice the other halves into 8 slices each to create the spider legs. Place four legs on either side of the olive bodies. Store covered in the fridge until ready to serve.