because its an auto attack modifier, not a usable spell.
off the
wiki
Toxic Shot adds an on-hit effect to Teemo's basic attacks.
Does not apply spell effects.
Magic damage will be mitigated by magic shields.
and for comparison purposes only
Vorpal Spikes adds an on-attack effect that causes Cho'Gath's attacks to launch a linear, pass-through skill shot in his target's direction.
Applies spell effects as an area of effect ability.
Spell vamp is reduced to one-third effectiveness.
Rylai's Crystal Scepter will apply a 40% Slow icon slow for 1 second.
Magic damage will be mitigated by magic shields.
Spell shields will not block the ability. (such as banshees veil)
TLDR on chogath, his spikes can miss his target if they aren't the focus of the auto attack