A Henagon has 1 side

A henagon has 0 sides.

it is called a henagon is a one sided shape

Ingeometry a henagon (or monogon) is a polygon with one edge and one vertex. It has Schläfli symbol {1}. Since a henagon has only one side and only one interior angle, every henagon is regular by definition. In Euclidean geometry, a henagon is usually considered to be an impossible object, because its single edge would extend to infinity. However, in spherical geometry a finite henagon can be drawn by placing a single vertex anywhere on a great circle. Two henagons can be used to construct a dihedron on a sphere, with Schläfli symbol, {1,2}.

it looks like this -