The ornamental design for a decoyanchor, as shown and described.
FIG. 1 is a side view of the decoy anchor showing the new design in a state of use;
FIG. 2 is an enlarged perspective view of the bottom portion with weight element, shown broken away for convenience of illustration;
FIG. 3 is a partial cross-sectional view of the weight element of the anchor line; and,
FIG. 4 is a side view of the design showing the weight element in alternate positions.
The broken line showing of a duck and weight elements are included for the purpose of illustrating environmental structure only and form no part of the claimed design.