It would probably have been a trigon, but that is besides the point. The name triangle came first and was widely accepted.
You also have quadrilateral rather than tetragon.
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triangle
It is called a "triangle" I know, not very imaginative is it?
A triangle with no equal sides is called a Scalene Triangle.
It is an equilateral triangle
this is called an obtuse triangle the triangle that has one angle 90 degrees is called a right traingle a triangle that has 3 angles less than 90 degrees is called an acute triangle