Dodecahedron
Icosagon is a 20 sided figure.
A dodecahedron.
pentagonal pyramid
Icosagon
I believe it is tridecahedron.
2D is a decagon. 3D is a decahydron.
A twenty-eight sided figure is called an Icosikaioctagon. The word icos refers to the number twenty and the word octagon refers to eight. When combined, they form the word Icosikaioctagon.
A 20-sided flat figure (polygon) is called an icosagon. If it is a solid figure--three-dimensional--it is an icosahedron.
There are many possibilities, and here are a couple: A dodecagonal prism (dodecagon = 12 sided plane figure); A heptagonal pyramid (heptagon = 7 sided plane figure).
There is not a definite name for a 3D figure with 18 sides.It is possible that it is something else - but the information to test this is not provided.
A 2D seven sided figure is called a heptagon. A 3D seven faced polygon is called a heptahedron.
A twenty sided, 2D figure is called an icosagon.