The ornamental design for a rotatable pet anchor, as shown and described.
FIG. 1 is a side view of a rotatable pet anchor showing my new design.
FIG. 2 is another side view of the rotatable pet anchor.
FIG. 3 is a top view of the rotatable pet anchor; and,
FIG. 4 is a bottom view of the rotatable pet anchor.
The design has been shown broken away in the middle in FIGS. 1 and 2 to indicate indeterminate length. The broken line showing of environmental structure is for illustrative purposes only and forms no part of the claimed design.