You should keep them in a pen, and protect them if they're in a dangerous environment.

I had some in the plains, behind double stone walls, and a fuling got in somehow and killed all but a single piglet. After growing them back, a deathsquito wiped them all out.

If you're in the meadows, just put a cheap wooden fence around them to keep them together and it should be fine. You do need to "harvest" them or else they will not produce anymore.