He is using a proverb to talk about people who at one time confessed to be christians who returned to their sin (vomit) and explaining why they returned to their vomitPeter is using a Proverb that is not speaking about sheep but about habits.
because they are dogs.
A dog will always return to his vomit. like a non believer will always return to his sin. John said whoever sins (of the world. eats their vomit) has never met God, whoever is born of God can not sin (go back)
why do you reject scripture?