The ornamental design for a teether, as shown and described.
FIG. 1 is a front perspective view of a teether, showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a right elevation view thereof;
FIG. 5 is a left elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The even-length broken lines in the drawings depict portions of the teether that form no part of the claimed design. The dash-dot-dash broken lines in the drawings indicate boundaries of the claim.