The ornamental design for a lifejacket for an animal, as shown and described.
FIG. 1 is a right side view of the life jacket for an animal of our new design in the closed position;
FIG. 2 is a left side view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a top view of the life jacket for an animal in the open position showing the exterior surfaces of the straps extended away from each other and in the same plane;
FIG. 6 is a bottom view of the life jacket for an animal of FIG. 5; and,
FIG. 7 is a front view of the life jacket for an animal in the closed position.