Nostalrius Begins - Quality wow vanilla realm (1.12)

Warlock pet(s) do not attack when i cast Corruption or any Curse, only when i cast direct damage spell (Shadowblot, Immolate).

This is NOT a bug. This is how vanilla wow was designed. First, you need to toggle "auto cast" on for your pet's ability (in this case, my assumption is your imp's firebolt). Second, you have to actually TELL your pet to attack your target. Default hotkey for this is CTRL+1.

It doesn't make sense the pet auto-attack just fine when i cast immolate but now with DOT spell.
It will also attack if i click right button (auto attack) even when im out of range.
IT works correctly on other Vanilla servers.

In addition, a defensive pet will immediately attack any mob which the hunter attacks.

http://www.wowwiki.com/Defensive

My wife and I came across this last night. I have to agree with drevan, as I tried a few other vanilla servers before this one where the pet auto-attacked when the spell hit the mob. I mean, I suppose I could make a macro to tell pet to attack then cast my spell, but this should be fixed.

One way to solve this problem is to write a macro that binds together pet attack and curse of agony - as an example.

/script PetAttack(target)
/cast curse of agony

With this macro now your pet will attack anything you cast curse of agony on. Keep in mind that your pet will then change target anytime you cast curse of agony using this macro.