Vertical asymptotes usually occur when there is a zero in the denominator of a fraction, e.g. tan x at x=pi/2.
Horizontal asymptotes only occur when x is approaching ±∞, e.g. e-x as x -> ∞
What you have called slant asymptotes occur when some terms in an expression become relatively small by comparison to another term. e.g. (2x2+3)/(x+5) as x -> ∞.; another good example is Stirling's formula for n!
Please note that in what you have called a slant asymptote only the relative difference becomes small; the absolute difference increases without bound.