There is no specific name.
Using a single straight line, a rectangle can be halved into triangles, smaller rectangles, or pentagons. There are infinitely many possibilities using more than one lines in the cut.
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hexagon
Such a polygon is said to be a "regular" polygon, meaning that all its sides are equal and all its angles are equal, so we have; equilateral triangle; square; pentagon; hexagon; heptagon;octagon;nonagon;decagon - to give but the first nine possibilities. The next one would have 11 equal sides and angles, and I don't know its name.
It is called a quadrilateral. This is the general term for a four-sided polygon, but if at least two sides are equal or two angles are equal, it has a more specific name.
Equilateral triangle.Also a square.Also a regular pentagon.Also any regular polygon of any number of sides.A shape with equal sides and angles is a called a regularshape.[But there are also other shapes, such as a rhombus, which has equal sides but is not a regular polygon because it does not have equal angles. There is no collective name for such "squashed" polygons".]
There is no specific name, except in the case of a triangle: scalene. You should not call it simply an irregular polygon, because a regular polygon implies that ALL sides are equal and ALL angles are equal. So a rhombus, where all four sides are equal, is still irregular.