Flame Body is an ability possessed by Fire type animals.
In Battle Use
Animals with the Flame Body ability have a 30% chance of inflicting the Burn status onto their opponents if they are hit with a physical attack.
Out of Battle Use
Animals with this ability can warm up eggs when placed near the eggs.
Fire type animals likely activate this ability as a form of defense or when engulfed in flames. The way the ability is also able to hatch eggs may be reminiscent as to how birds keep their eggs warm in their nests.