A site gives $x^2 - y^2$ for a hyperboloid-lie 3d graph, what about z? Is there a name for this 3d graph?

Question

A site https://www.math3d.org/ renders this 3d graph as its homepage

and gives this formula for that graph

$x^2 - y^2$

What about z? What is the full formula for that graph? Is there a name for this 3d graph?

Answer 1

$$ z= f(x,y)=x^2-y^2 $$

is a Monge plot you have shown for the hyperbolic paraboloid, aka hypar. It has negative Gauss Curvature; what is shown at the centre is one among Saddle Points.