A couple of things that aren't accurate from some of the posts. Even starred boars don't do only night spawns so despawning at day break doesn't happen with them. Starred wolves spawn only at night and they do despawn at day break UNLESS they have started taming. I am certain of this because my first one starred wolf started taming, reached 27% and escaped its enclosure. 3 game weeks later I found a 1 star wolf during the day in the same spot that it had initially spawned. It was 27% tamed so it was that same wolf. I got it back to the enclosure and finished taming it.

If you don't have full workbench coverage mobs can spawn inside your walls (and deer and birds can spawn in even with full workbench coverage).
https://steamcommunity.com/sharedfiles/filedetails/?id=2920176358

If your base is in the meadows and you lack full workbench coverage only greylings can spawn in during the day but at night greydwarves can spawn in. If the incomplete workbench coverage is inside the pen a greyling during day and/or a grey dwarf at night could spawn inside the pen and kill the boar.

If the incomplete workbench coverage is inside the base but outside the pen a night time spawning greydwarf could throw rocks at it until it died.

If you found boar meat and leather scraps that's probably the answer.

The other thing that happens frequently is that the boars can clip through walls and enclosures. The boar sprite is larger than the walls and the animals load in first when you enter the area and the walls load in later. If the boar was moving when that happened it can be outside the walls. In that case it will have returned to its original spawn point. If you go there you may find a partly tamed (but still aggressive) boar. You can then lead it back to your pen and complete its taming.

The clipping issue is why I keep my boars inside a barn, that is inside a fence, that is inside my exterior walls
https://steamcommunity.com/sharedfiles/filedetails/?id=2853688746
https://steamcommunity.com/sharedfiles/filedetails/?id=2853688768