A dog, or any other animal, running on to the field could mean a dead ball or a boundary, but this is at the umpire’s discretion.
The Laws of Cricket cover any animal, person or object coming on to the field, though of course this doesn’t cover the umpires, permitted players or playing equipment. Someone or something coming on to the pitch doesn’t automatically have any effect or result, particularly if it doesn’t impede the players or affect play.
If the ball hits a person, object or animal, the umpires can award a boundary (four or six) if they believe the ball would otherwise have reached the boundary. In this situation they are technically choosing to treat the person, object or animal as the boundary. (However, if the boundary rope is moved out of place, the umpires must treat the boundary line as being in its original position.)
If a person, object or animal is on (or above) the field and the umpires believe this disadvantages one side, they must immediately call the ball dead. This means no more runs can be scored and the batters can’t be dismissed. Normally the ball will count as one of the usual six an over unless it happened before or during the bowler’s deliver and thus the batter never had a chance to play it.
Note that this doesn’t cover the ball hitting the permitted players or umpires. An “obvious” boundary being blocked by the ball hitting an umpire is simply bad luck for the batters and, indeed, the umpire.
In the unlikely event that a ball hits an animal, object or person before hitting the ground and then is diverted over the rope, the umpires have three options:
- Consider the animal, object or person as the boundary and award a six.
- Take no special action and thus award a six.
- Consider the ball dead at the point it hit the animal, object or person because it unfairly disadvantaged the fielding team. The batting side would thus only receive any runs the batters had run.