Does what it says on the tin — animals randomly invading the fields of various sporting events. Dogs get pride of place, and seem to think they're participating. But it's not just dogs.
Here's a squirrel that started stealing bases during a Chicago - Cleveland baseball game:
This chicken — apparently a Blackburn Rovers fan — gets involved in a Blackburn - Wigan soccer match:
This urban fox managed to get some air time prior to a 2011 Six Nations rugby match between England and Scotland in London. The poor fella looked pretty agitated, like "where the hell did all these people come from?" But it managed to get off the field safely.
This pine marten gets involved in a Swiss soccer match and gives two players something to think about.
This cat strolls out into the middle of a Liverpool - Totenham Hotspur soccer game like it owns the place.
EDIT: procastinationathon noted the egregious lack of goat on this list, and offered to remedy that issue with a goat playing goalie in Greece.
Here's a pup that got involved in a 2013 Copa Libertadores match:
A couple of puppies play tug with a bag they found, right out into the middle of a Galatasaray - Aalen soccer match in Turkey:
This dog starts out by chasing some paper airplanes that were thrown onto the pitch during this Argentina Primera B Nacional soccer match, and ends up defending the midfield. He just needs to learn some structure (and bladder control).
A very friendly black lab decides to go say hi to a player in the middle of an Argentine Primera Division match in 2014:
Sometimes you just need to scratch your back, like this terrier did in the middle of the pitch during a Colombian pro soccer match.
A police dog gets off-leash and attacks the ball, is relegated to desk duty:
This lab makes fools of the goalies in a York City - Bristol Rovers soccer match:
But it's not just soccer matches. This pup decided to get involved in a field hockey game:
This terrier who invaded the pitch in Dublin during a 2004 International Rules football match between Ireland and Australia. The dog tries to get into the game, going after the ball when the players did and avoiding defenders like a pro. There's a bit of a time lapse in this clip, because the dog was involved in the game for almost five minutes. When the dog was finally caught by the stewards (with what looks like a big grin on his doggy face), the crowd in unison booed the stewards.
There are loads more out there if you look for them. South American soccer pitches in particular seem to be really popular canine hangouts.