If he posted a pic of the animal for sale, and sent you a different animal, then it's fraud. Tell him so, show him a pic of the animal you recieved with the pic he had in the ad, and point out definite differences. Tell him it is fraud, and you want this situation resolved IMMEDIATELY.
Try to sound professional but firm, without calling a bunch of names and not threatening the guy.
If you threaten to go beat him up, that's call for HIM to legimately call the cops on YOU.
If he refuses to make it right, go to the BOI, and post it in full.