We claim the ornamental design for an accessory for a toy robot, as shown and described.
FIG. 1 is an isometric view, from the top left, of the accessory;
FIG. 2 is an isometric view, from the bottom right, of the accessory;
FIG. 3 is a plan view of the top of the accessory;
FIG. 4 is an elevation view of the right side of the accessory;
FIG. 5 is an elevation view of the left side of the accessory;
FIG. 6 is a plan view of the bottom of the accessory;
FIG. 7 is an elevation view of the front of the accessory;
FIG. 8 is an elevation view of the back of the accessory; and,
FIG. 9 is an isometric view from the top left of the accessory coupled to a toy robot.
The broken line showing a toy robot in FIG. 9 is included for the purpose of illustrating environmental subject matter and forms no part of the claimed design.