Mob Special Drop

If you're talking about doing this to a mob already in the game, I have no idea. But if you're asking about a custom mob that you've created, then continue on.

So what you're gonna want to do is make a new procedure and name it whatever you want, then you're gonna set it up something like this: https://ibb.co/bZr8CT

In my example, I used a diamond for the killing tool to give me the emerald from the mob. You can change these to whatever you want (killing tool replaces diamond, custom drop replaces the emerald.) After you do that, you're gonna want to go to the procedure screen of your custom mob and change the procedure for "When Mob Dies" to whatever you named this procedure.

Hope I could help.