Anyone know how to program special bullets?

I am looking for help in creating two types of special bullets: One that follows and entity until it hits, much like a homing missile; a bullet that will slow down the entity it hits; and lastly, a bullet that leaves some sort of mark on the entity when hit (similar to that one arrow in i believe 1.9 minecraft). I've tried to make a bullet give slowness to an entity, but the player gets the same effect, which i don't want to get.

Here's an example of what i mean for the bullets that follow an entity. 

As you can see, the bullets will move to hit the nearest person, but in my case, I want it to, after being shot, curve to the nearest entity that isn't the player that shot them. Could anyone with exceptional coding knowledge be able to help me out?