Originally posted by sneakyweazel:

One thing about how-to not lose all your herd in a raid/to random grounded enemy:
Build secondary but somewhat bigger perimeter around the pen. For example I have few meters between these defensive lines and the mobs don't notice the herd nor is the herd alerted by a wondering mob so nobody aggros at nobody and usually nothing attacks the fence.

This is good advice. That's how my first pen of boars got free. A mob wandered by and spooked the boars. The boars busted down the little pole fence. I come home to boars in the kitchen. The outer perimeter fence was built the next day. Corralled who I could and made sausages out of the others.