That means a heart curve.
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A cardioid has 1 line of symmetry.
A cardioid is one possibility. See link, below.
Isosceles triangle, kite. -improvement: an isosceles triangle actually has three lines of symmetry, equal to the number of sides. It is easy to design irregular polygons with an odd number of sides that have just one line of symmetry. A parabola. Many more functions that are more complex - cardioid, for example.