They're small and furry (or scaly, or feathered) and, for the most part, non-threatening, unlike the grown men and women who sometimes produce similar antics.
When animals storm the field/ice/pitch/court during a game, the camera crews don't immediately cut away from the action. They revel in it. Why wouldn't they? It's cute!
And plus, seeing the players attempt to combat animals gone wild makes for some of the best TV of any given year. These guys may be athletic, but you wouldn't know it from watching them try to catch a loose pig.
Here's a look at some of the most adorable (or hilarious) field stormings ever.