How to mark unique mobs?

Hey! I'm trying to figure out a way to uniquely 'mark' a mob. For example, there are two creepers, and I want one creeper to be marked creeper A and the other creeper B when I right click them. That way when I interact with the mobs in the future, I can code different events to happen to them based on if they are creeper A or creeper B. Is unique mob marking possible?