The ornamental design for an eggcostume, as shown and described.
FIG. 1 is an front view of the egg costume design, which embodies my design;
FIG. 2 is a back view of the egg costume design;
FIG. 3 is a left side view of the egg costume design;
FIG. 4 is a right side view of the egg costume design;
FIG. 5 is a top plan view of the lower portion of the egg costume design;
FIG. 6 is a bottom plan view of the lower portion of the egg costume design;
FIG. 7 is a top plan view of the upper portion of the egg costume design;
FIG. 8 is a bottom plan view of the upper portion of the egg costume design; and,
FIG. 9 is an isometric view of the egg costume design in a position of use. The broken lines showing of the human being in FIG. 9 is for illustrative purposes only and forms no part of the claimed design.