During combat, large creatures have the potential to be dangerous merely because of their mass and the proximity of that mass in relation to defenders. A mastodon that weighs five tons, for example, could cause damage merely by shifting its bulk into the body of a defender, regardless of the defender's armor class or mass. The damage caused by huge creatures blundering around, then, is called "incidental damage," or damage that results without intention of the creature.
For any creature of more than 1,000 lbs., there is a 50% chance of that creature causing damage to any defender that is within one combat hex, or 5 feet, of the creature at any time during the creature's movement. Thus, if the creature passes through or directly adjacent to any combatant of species other than the creature, a 1 in 2 chance is rolled to see if incidental damage occurs.
That damage can then be as high as 1 possible point per 1,000 lbs. of the creature. A mastodon, to use the above example, weighs 10,000 lbs., for a maximum of 10 damage. A random die roll is made, therefore, of 1 to 10 (or 1 to x, whatever the maximum damage is).
At a certain point, only so much mass can effectively come into contact with others, so there is an upper limit to the amount of incidental damage per person per round: 1d12.
Both the chance of incidental damage and the roll for damage, when incidental damage occurs, is made per massive creature that passes, moves or stops within one combat hex of the defender.