The ornamental design for a dog collar, as shown and described.
FIG. 1 is a front perspective view of a dog collar showing my new design, the broken lines shown are for illustrative purposes only and form no part of the claimed design; and,
FIG. 2 is a front elevational view thereof, it being understood that both ends are shown so as to represent indeterminate length.
The broken line showing of the dog in FIG. 4 is for illustrative purposes only and forms no part of the claimed design. Both ends of the collar in FIGS. 1-3 are shown so as to indicate indefinite length.