A pyramid.
A three-dimensional shape with eight triangular faces is called an octahedron.
An octahedron
square pyramid
the sides of a pyramid are triangles
This is an Octahedron!
There is a contradiction in your question. If the base has four equal sides then is it a square so not all the faces are Triangles. However if the base is a square and the rest of the faces are triangles then your object is a "Square-based Pyramid"
A pentagon
an octagon
2 equilateral triangles
The answer is a pyramid.
octagonal pyramid :)
The answer to your question, is an Octahedron.