Stir together rice, vinegar, sugar and salt in a large bowl, until well mixed. Place about ⅓ cup measure rice (packed lightly), on a silpat and pat into about a 4-inch square.
Place a strip of cucumber, bell pepper, carrot and avocado in the middle. Fold in half with the help of silpat, pressing firmly.Using damp hand, shape the roll into an egg shape with a slightly end for the face. Insert a thin scallion green in the fat end (for tail).
Insert 2 small triangles of cucumber (cut from the slices) for ears and place black sesame seeds for eyes.Repeat with remaining rice. Serve on a bed of Nori seaweed and soy sauce for dipping.