After making about 3 dozen boar 'pens' I've settled on pits dug into the ground. 100% of ground level pens, regardless of fence type, have ultimately had tamed boars/wolves running around my base. Even with dug out pens it is necessary to make the walls tall enough or they can stack themselves high enough to get out. Wolves do this too. Just watch when an enemy is nearby and you'll likely see them forming pyramids 3-5 layers deep. It looks like an "Americas Got Talent" performance.
Starred critters spawned at night will usually despawn at dawn unless they are actively being tamed with your player in the area. Happy ranching!