The ornamental design for a teether, as shown and described.
FIG. 1 is a top view of the teether;
FIG. 2 is a front view of the teether, with interior portions of a pair of side rings illustrated in broken lines, which interior portions do not form a part of the claimed design;
FIG. 3 is a left side view of the teether, the left side view being a mirror image of the right side view of the teether; and,
FIG. 4 is a bottom view of the teether.
FIG. 2 of the drawings includes a broken line depiction of the interior portions of a pair of side rings of a teether, which interior portions do not form a part of the claimed design.