I claim the ornamental design for an adjustable animal garment, as shown and described.
FIG. 1 is a front view of an adjustable animal garment showing my new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a right side view thereof; and,
FIG. 4 is a left side view thereof.
The broken lines showing the hood in FIGS. 1-4 depict portions of the adjustable animal garment that form no part of the claimed design. The remaining broken lines depict stitching that forms part of the claimed design.