Warthogs are found on large islands. There are 0 - 2 warthogs per large island. Warthogs will charge at you and, if they connect with you, they will deal damage and pick you up and carry you to the end of their charge. If you avoid their charge they will continue for some distance before skidding to a stop and recovering.
The safest method to kill the warthog is to shoot them from on top of a rock with a bow so that they cannot reach you, however, not every island will have a safe place to stand. The other way to kill warthogs is by using a spear and strafing around their charge. You can wait for a warthog to charge you and strafe around it by either jumping or stepping out of the way as they are coming at you because they are not able to change directions once their charge has begun. You will not be able to do this without some distance between you and the warthog, though. After the warthog has charged past you, you can run up to it and get 1 - 2 stabs on its butt before it recovers and can charge again. You can stab it more times, however, you will not have enough time to get away before it charges again and deals damage. It will take about 10 stabs with a metal spear to kill a warthog.