Fishtail teats are a breed fault, and can pop up many generations later.... seemingly out of the blue. I personally would start a new line by going to get a kid from a reputable breeder, probably what is a (white) AOV (HB). If you have milk to bottle feed you can take them quite early, and check out the udder etc of the dam. Then stop breeding from your existing goats and eat/pts. But there are no guarantees it won't come back later,, males can carry it as well, so always worth (if sometimes impossible) to check out his teats too...!