Originally posted by se05239:

Me and a pal setup a pen, following a quick guide on the internet (since we're both beginners and clueless on what to do) and after we managed to round up some boars to put in the pen and feeding them.. we went out doing chores. I went out hunting boars for meat and my pal portal'd out to work on a storage house at our "main" base and when we came back, all five boars had teleported outside and sprinted away into the wilderness.

How do you.. make them not phase through the walls? The pen is made with normal wooden walls and doors.

As said, work with multiple walls. Also, the bigger the enclosure, the less chance they are close to the wall.