Plot3D[x-x^3/8-y^2/2,{x,-4,4},{y,-4,4}]

Plot3D[y^2-x^2,{x,-4,4},{y,-4,4}]

Plot3D[Sqrt[36-9x^2-4y^2]/3,{x,-3,3},{y,-3,3}]

ContourPlot[y^2-x^2,{x,-3,3},{y,-3,3}]