To be honest and this is from my personal experience, I think all sows have it. I think its to the internal muscles that squeeze the poops out but when young its more obs'. I have just checked two adult sows and they have it but the vulva (external bit of skin) is larger than a young sow so it covers it over.
Also, tries not to get too rude, the vaginal entrance is also down there so it could possibly be a covering for that because the anus and vaginal entrance are pretty close.
So it could be one of the above possibilties, TBH noone has ever asked me what it is.
Emma x