The ordinary tetrahedron, or triangular pyramid, has no diagonals — every pair of vertices is joined by an edge. How many other polyhedra have this feature? In 1949, Hungarian topologist Ákos Császár found the specimen above, which has 7 vertices, 14 faces, and 21 edges.
But so far these two are the only residents in this particular zoo. The next possible such creature would have 44 faces and 66 edges, but this isn’t realizable as a polyhedron. Whether there’s anything beyond that is not known.