A'mule' error occurs when one side of a coin has been struck with the wrong die. Your 2014 Year of the Horse 1oz Silver was mistakenly struck with the obverse from the 1oz Britannia.
You can see the difference as the rim of the error coin features the distinctive denticles that characterise the Britannia series - yet the rim is supposed to be plain. We have here graded examples of both the coins involved. Coin errors, especially on United Kingdom issues, are exceptionally uncommon. In the stamp world errors of printing have long been sought out above all other issues.
This one mistake will mark your coin out in numismatic history as one of only a handful of times where the Royal Mint have released a mis-struck coin to the public.