The ornamental design for a castor, as shown and described.
FIG. 1 is a top, front and side perspective view of a castor showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a side elevational view thereof from the left of FIG. 2, the side elevational view thereof from the right of FIG. 2 being the mirror image thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a top plan view thereof; and
FIG. 6 is a top, front and side perspective view of a second embodiment;
FIG. 7 is a front elevational view thereof;
FIG. 8 is a side elevational view thereof from the left of FIG. 7; the side elevational view from the right of FIG. 7 being a mirror image thereof;
FIG. 9 is a rear elevational view thereof; and,
FIG. 10 is a top plan view thereof.
The bottom plan view forms no part of the claimed invention and so is not shown.