Them start spawning at 800m away from center map ( i just got lucky of getting one in that limit in my 2nd world) , or as someone said , in boar runestone with 1% chance , in runestones spawns at start 4-9 boars , and if you want to keep respawning you have to kill all boars spawned , if you save one the runestone stop working ( happen to me in same 2nd world where i get 2nd 2* boar but the runestone stop working )