Targeting system

So i was working on my server and i have a problem, when i summon like a zombie i want to be able to control what target it attacks, so i can like set
/summon ~ ~ ~ zombie{Target: !@s, @a}
Or it could be a attribute so you can like change it when the zombie is alive, if this is somehow possible without plugins then sorry but i would also want to know how since google search doesnt really help