The regular tetrahedron - one of the 5 platonic solids.
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A parallelogram.
In an octagon, there are 20 equilateral triangles. Each side of the octagon can form two equilateral triangles with the adjacent sides, resulting in 8 equilateral triangles. Additionally, the diagonals of the octagon can form 12 more equilateral triangles. Therefore, the total number of equilateral triangles in an octagon is 8 + 12 = 20.
tetrahedron
A four sided figure would be a tetrahedron and a 20 sided would be an icosahedron. They both have equilateral triangles for faces so your question is a little incomplete.
Isosceles TetrahedronA solid with four faces is a tetrahedron. Each of the faces is a triangle. If all the triangles are congruent, you have an isosceles tetrahedron.