The ornamental design for a dog's bath mat having a removablesponge, as shown and described.
FIG. 1 is a front perspective view of a dog's bath mat having a removable sponge with the sponge inserted in a cavity in the mat showing my new design;
FIG. 2 is an enlarged right side view of a dog's bath mat having a removable sponge;
FIG. 3 is an enlarged left side view of a dog's bath mat having a removable sponge;
FIG. 4 is a rear view of a dog's bath mat having a removable sponge, the handle shown in a partially collapsed position;
FIG. 5 is an enlarged top view of a dog's bath mat having a removable sponge;
FIG. 6 is an enlarged bottom view of a dog's bath mat having a removable sponge; and,
FIG. 7 is an exploded front perspective view of a dog's bath mat having a removable sponge showing the manner of assembly with the sponge removed.