Originally posted by Altimely:

Gotta be far from the spawn (some say 800 'meters', can't confirm) and your best bet is around a Boar rune stone. Not guaranteed, but a better chance.

I read this was the case as well, but there was a boar stone roughly 120-150 "meters" from my spawn on my last seed, and I was able to get a 2-star boar from it. (One of my players found it, and I didn't believe it until I saw it myself.)