There are at least two documented cases where turtles were found with constricting plastic rings around their midsection. One with a six-pack ring, another with a milk bottle safety seal ring:
There is no evidence that these are hoaxes, nor that some other process caused the deformity. It is reasonable to assume they are solely a result of the plastic rings they were found with.
Peanut, the figure 8 turtle, appears to still be alive and is living in the Busch Conservation Area in the Missouri Department of Conservation. This turtle is used in outreach programs to teach students and other groups the dangers of plastics in the environment, and there exist many reports and images of this turtle over the years. It is very unlikely that it is a hoax of any sort:
The story of Peanut the Turtle at the Minnesota Department of Natural Resources.
Turtle Cut Free From 6-Pack Rings Is Unstoppable 20 Years Later