Wolf boss in first zone vanished. Never got to kill him.

In the starting area, challenging myself by playing on nightmare only. BTW what all does nightmare do? It's not clear at all what the benefit to increasing difficulty is. Anyway, I'm in the cave that has the wolf boss on the first island. He wrecked me obviously because it's on nightmare. So I built some walls in the cave and the wolf started glitching out. Running around a bunch then healing all the damage. So I reposition to get another spot and next thing I know the wolf runs around the corner and vanished. His icon still on the map, but the wolf is nowhere. So I exit and reenter the game after moving a bar far away. I go back to the cave and some of the small wolves respawned but boss is gone. I go back to my base and rest, and boss is still gone. Please help. Thanks.