How to Make It
Cut a small curved wedge from 2 sides of each cucumber slice to create a skull shape using a pairing knife. Poke holes in the top (wide end) of the cucumber to create the eyes using the flat end of a wooden skewer. Draw three thin lines below the eyes to create the mouth using the pointy end of the skewer.
Combine the vinegar, chile, sugar, 2½ teaspoons salt, and ½ cup warm water in a bowl. Whisk until the sugar is dissolved. Add the cucumber, carrots, yellow squash, and fennel. Refrigerate for at least 24 hours and up to 1 month.