''sack'' hanging from my dogs butt?

i recently took a rescue dog in, and they claimed he was neutured. At first i thought maybe he just had one ball, because they either have 1 or none. but this is too far up to be one of those. it is perfectly round and looks like a nut, but i think it may be a cyst. he's bloodhound, so when he sits he acts like it hurts, but i thought it was just because he was so big (100 lbs) i'm gonna have to bring him to the vet, but was wondering if anyone had a guess as to what it may be?