There are a few ways to go about hiding the tail on a knitted animal. One way is to simply sew it onto the back of the animal, either by hand or with a sewing machine. This will keep it securely in place and prevent it from coming loose.
Another way to hide the tail is to stuff it inside the body of the animal. This can be done by threading it through the body before stuffing, or by carefully stuffing it into place after the body is already stuffed. Either way, this method will keep the tail hidden away safely.
Finally, you could also attach the tail to a piece of yarn or string and then tie it off somewhere on the back of the animal. This might not be as secure as other methods, but it can work well enough if done carefully.