(A) \( f^{\prime}(x) = 8x^{3} - 32x \)
(B) The slope of the graph of \( f \) at \( x = -3 \) is \(\boxed{-120}\).
(C) The equation of the tangent line at \( x = -3 \) is \(\boxed{y = -120x - 336}\).
(D) The value(s) of \( x \) where the tangent line is horizontal are \(\boxed{x = -2, 2}\).