How to make an already existing Doom monster drop a new item?

[Doomsday]

Also how to make that item only have a chance of dropping from the enemy. I'd rather not make a clone of a monster just to give it something new to drop. 

[Nootrac4571]

If you're targeting ZDoom/GZDoom, then you can use DECORATE to modify existing monsters.

 

For example, this would give the cacodemon a 50% chance of dropping a clip:

 

actor Cacodemon2 : Cacodemon replaces Cacodemon
{
    DropItem "Clip", 127
}

 

https://zdoom.org/wiki/Actor_properties#DropItem