The British One Pound coins have the date on the obverse.
They are not mintmarked since they are all minted at the Royal Mint, ie. there is no need to make a distinction.
A Sovereign is Sterling coin. In 1880 a Sovereign was worth a Sovereign or One Pound. It's face value has not changed since it was reintroduced into the currency in 1817.
It's worth exactly one pound.
To date, no horse has ever appeared on a British One Pound coin. There has been a unicorn as part of the Royal Arms on the 1983, 1993, 1998 and 2003 One Pound coins.
It doesn't matter what date - up until decimalisation, the pound sterling was always equal to 20 shillings.
Assuming USA coin - question does not say. Not all have mint marks. If there is one it is above 72 of date.
As of January 21st 2011 about 1.60 dollars. Please see related link for up to date information.
All British Two Pound coins have a date on them somewhere. The coin to which you possibly refer is the 1995 50th Anniversary of the end of Second World War, with a dove on the reverse. The date (1995) is on the edge.
Assuming the coin is circulated and has no mint marks, retail values are $17.00-$26.00 depending on the grade. 1884 is a common date for Morgan dollars.
No US silver dollars were struck in sterling (92.5% pure) silver alloy. Circulating dollars were struck in "coin silver", which is a 90% alloy, while American eagle bullion coins are 99.9% pure. Please check your coin again and post a new question with its date and country of origin.
If the "mint marks" on the reverse are the letters "F" and "G," then these aren't mint marks at all, but the initials of Frank Gasparo, the designer of the half dollar's reverse. In that case, you have an ordinary 1989-P half dollar, which will run between $1 and $90, depending on the coin's condition, as of May 27, 2007. If the "mint marks" on the reverse are something else, then your coin is best evaluated by a professional coin dealer who specializes in error coins.
You do not specify the "date", the nature of the "deformity", or the type of Two Pound coin. Any coin with a "genuine" minting flaw would have some value, above the usual, as a collectible coin. Genuinely flawed coins are not necessarily known about or documented until somebody turns up with one, since they are an "accident" of the minting process, and have escaped detection during quality control at the mint therefore, a valuation cannot be anticipated. A reputable coin dealer should be able to identify and confirm the coin as genuine and make a valuation.
No US silver dollars were struck in sterling (92.5% pure) silver alloy. Circulating dollars were struck in "coin silver", which is a 90% alloy, while American eagle bullion coins are 99.9% pure. Please check your coin again and post a new question with its date and country of origin.