Although the fifth leg didn't seem to bother the pup, Siegel intended to have it removed, but was shocked to learn it would cost another $2000 to do so. Fortunately, a Manhattan vet was so touched by Siegel's actions that he offered to remove the leg free of charge.
“She is beautiful, she’s not a freak, she’s a normal little puppy dog and she should be just like all the others,” Siegel said.
Precious was born with a fifth leg protruding from her stomach, between her hind legs. The extra leg has six toes.
Link Via Dogster Dog Blog
He wasn't treated worst as a sideshow freak than other performers. However, his boss was indeed a jerk and took a way most of Merrick's money. He was badly treated mostly by townfolk he encountered in the streets, not people who came to see him when he was exhibited. You have to keep in mind that the movie wasn't true to reality ;)
A dog, working today in the sideshow industry, would be well treated, not just because of PETA and other groups working for the ethical treatment of the animals, but because he would probably be loved by his fellow freaks and performers.