Two good friends of mine have experienced them. Both waited to deal with it, a weeping growing abcessed cyst on the tail bone, till they could hardly walk. Looked like they were starting to grow a tail.

Both had surgery, wound left open packed with medicated gauze to drain, and were sent home the same day. Very painful. I changed the dressing for one of them once. I was shocked they sent him home like that. The opening was large enough I could have stuck my fist into it up to my wrist. I've seen deer shot during hunting season that didn't look as bad as that.

Don't know the cause, but I feel sorry for anyone who has to suffer with it.