An equilateral triangle.
A regular hexagon.
It is a regular 8 sided octagon.
an equilateral triangle
An irregular octagon.
a hendecagon has 11 sides =)
A 5-sided figure is called a "Pentagon".
A congruent triangle is a triangle with all its sides being unequal to each other.
a 12 sided figure is called a dodecagon
A scalene triangle
An 11-sided figure is a hendecagon, or an undecagon. Either name applies.
That doesn't have a special name. If all the angles are also congruent (and convex), then it is called a regular polygon.
A Dodecahedron has twelve pentagonal faces. A Dodecagon is a plane figure with twelve sides.