Originally posted by Draconifors:

The boar is too far from where you found it, probably. Every time, after H&H, I've brought a boar from farther than the render distance it's kept running back and forth along the wall closest to its roaming location.

This happened to me as well, except that the pen was just a few meters from their spawning location (near a Boars runestone), and I thought it was the reason. The other pig was normal though. I killed it once they reproduced.