Shetlands should have a very distinct, fluke-shaped
tail--broad and wool covered at the top tapering down to a hair-covered tip. 

The length of the tail can vary somewhat but should be in proper proportion to the size of the sheep and not exceed 6" in length.  13 vertebrae are present in a Shetland tail.

  The tail to the right is too long.
 

Some Shetlands have "rat" tails--narrow from top to
bottom and often completely hair-covered.  These tails
tend to be overly long as well.  This type of tail can
be corrected fairly easily through proper breeding
practices, if the ewe exhibits good Shetland type in
all other respects.
 

Another correctable fault is a tail that doesn't taper
down to a point, but in all other respects is
correct.

A Shetland with a fat/wooly tail shouldn't be used for
breeding registered stock.  This fault is very hard to
eradicate and can pop out in subsequent
generations.