Pigfoot is both a boy with one pig foot and an urban legend.
The Legend of Pigfoot
In the 1970s, there was a farm boy who loved to dance. So he danced at school. The principal disapproved of this, so the boy was expelled.
But one day, there was a snowstorm. The boy tried protected his pet pig but that didn't work. So they were both swept into the snowstorm. Later, they ended up in a giant snowball. But, the boy fused with his pig and emerged with one pig foot and became known as Pigfoot.
This legend has become a trend in the Quads's school.
Nicky's Pigfoot Products
While the trend of Pigfoot was at its height, Nicky created numerous Pigfoot themed products. The most notable was a plastic bin of carrots he labeled "Pigfoot" and told costumers/students that it was candy.
The Real Pigfoot
Although the boy with the pig foot is an urban legend, at the end of the episode concerning Pigfoot a man comes to the Quads's school and tells them the real story behind Pigfoot.
He says that he was Pigfoot without a pig foot and that he really was expelled, had a pet pig, and survived the snowstorm but did not fuse with his pig. Dawn mentions that this part of the legend was an assumption.
Then, everybody dances.