What's the funny joke?

Pigs, horses, dogs, sheep and cats are all in a small boat, but the boat is broken and only one is left. So they decided to tell jokes. If they make all the animals laugh, they can stay in the boat, otherwise they will be thrown into the water. First, the horse spoke. After the speech, all the animals laughed except the pig. So the horse was thrown. Next is the dog. What the dog said is not funny. The other animals were indifferent, but the pig burst out laughing. The other animals were puzzled, so they asked him what he was laughing at. He said that the joke that Ma just told was so funny!