The Mechanical Critter ability

When this ability is used, the critter which it summons does not get attack by enemies that are near it unless they attack directly.

Now to the question, how do I get any unit that same ability in which the are not attack by enemies by default?

Note - I know its not the wander ability, and its not the critter itself.

I also thought about this. I still don't know. My guess is that the critter is a "Neutral Passive" unit, that you just can control.