Originally posted by Banzai Seppuku:

So do 2 star boar and wolf spawn as 2 star or do they become 2 star? I'm hunting for another 2 star boar now that I won't try to transport. Should I kill all the boars by the stones or do 2 star develop over time from some trigger?

I found a 1 star that was not by a runestone. So far breeding tame around a runestone has not produced any star piggies.

AFAIK there is no game mechanic for changing the number of stars a creature has after it spawns. Instead, stars are determined during the spawning process. If you are too close to the center of the map, normal boar spawns won't have stars. (but breeding works just fine)