It looks like the chick may have a prolapsed vent that has been pecked badly by the others. They will normally peck at anything red. Has the chick been constipated and is it passing normal droppings? Straining to poop can lead to a prolapse in chicks. A little chilled coconut oil cut into small peckable pieces can help a constipated chick. I would apply some plain Neosporin ointment to her vent twice a day and keep her near the others, but separated by fencing or a dog crate to give some time for healing. It may heal quickly. Hopefully the prolapse will decrease inside and go back in. For prolapses that are not injured, honey or a mild cream can help with swelling, keeping the prolapse tissue moist so that it does not dry out.