This is Peanut the turtle—Peanut the red-eared slider, to be exact. She was found in 1993 in Missouri and taken to a zoo in St. Louis where the six-pack ring was removed.
Today she is in the care of the Missouri Department of Conservation, and is doing well.
When she was a small turtle, she got the ring stuck on her shell and couldn't get it off. Over time, the rest of her shell grew but the area around the ring did not. The piece of litter had been around her shell for about four years when she was found.
If this had happened to a fleshy animal like an otter, the animal probably would have died from an infection. Since Peanut's shell protected her body, she was able to live with it, though some of her organs don't function properly.
Yes, her shell is hard and will not "pop" back to the round shape it should be.
If an item has any loops or openings it isn't difficult for an animal to get into trouble. The trapped animal often panics, making things worse. Since animals don't have the same dexterity as people, it's difficult for them to get out of entanglements.
Fishing line is a significant hazard, especially since it's often invisible. Entangled animals do not usually survive long—they either starve, or are quickly eaten by predators.
Small plastic bits and cigarette filters also pose a hazard to wildlife. Animals usually can't tell the difference between litter and food, so they eat the litter and choke, or fill their bellies full of plastic and starve.
*Information and photo courtesy of the Missouri Department of Conservation