The Suffolk County Society for the Prevention of Cruelty to Animals says someone stuffed a dog into a bag and then left it in the woods in Miller Place.
News 12 is told the animal was found dead and emaciated in November on Miller Place-Yaphank Road, north of Whiskey Road.
Officials say the animal was either a French of English bulldog and was only six months to a year old.
A $4,000 reward is being offered for information leading to an arrest.
Anyone with information is asked to call Suffolk SPCA at 631-382-7722.