I've never tried getting help to kill a boar. As for chasing you, some enemies may have less stamina, so have to give up, or their AI has some limit on how long they chase you (to simulate a lower attention span, etc). Also, some creatures may be designed to be territorial, and not stray too far from their original location. Undead may be stupid/single minded enough to follow you forever, and others may try to catch up as long as you remain in their sight range, but this could also be a bug.
