You could do it with events if encounters weren't random by using variables. You can easily make variables have random outcomes. These would not show up in the battle summary though. Alternatively, you could make multiple instances of the same monster except with different item drops and put them all into groups of varying composition.
If you wanted random encounters with conditional branching, then you coculd simulate random encounters through a parallel process event as well. So, random encounters with conditionals is possible if you do it that way.
If you want it to show up in battle summary, your best bet is to either find a multi-drop script or to make multiple groups with multiple monsters.