The ornamental design for a mermaidtail throw, as shown and described.
FIG. 1 is a front, top and side perspective view of a mermaid tail throw in accordance with my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a front elevational view thereof; and,
FIG. 7 is a rear elevational view thereof.
The dash-dash broken lines in the drawings depict portions of the mermaid tail throw that form no part of the claimed design. The dash-dot broken lines in the drawings represent a boundary of the claimed design and form no part of the claimed design.