Cloud and Horn of Fog: How?

Hi, does anyone know how to get cloud to work properly on non-hero units other than dragonhawk?

I tried to put (read: copy & paste) cloud to a random unit (with mana of course), towers werent affected
I tried to put horn of fog (the item equivalent of cloud) ability onto a random unit, towers still werent affected

but cloud works perfectly w/ dragonhawk and horn w/ heroes...

wtf?

Also it doesnt seem to prevent range or melee; i wanted to make something thats like disruption web in starcraft, but it just doesnt work. However if i make it prevent spells then it works perfectly. That and setting chance to miss will affect BOTH range and melee, and in my case I want mine to only affect just one.

Actually, it does prevent range and melee individually, but i dont think thats the case for chance to miss...

Anyway under attacks prevent, what is SPECIAL attack?