You need to make the tail fall off. If it did, and the Drake Sword is not in your inventory... try going to a Bonfire and try again... If the tail is permanently off... you are SOL! --- http://www.youtube.com/watch?v=-TsEpDS7-Mw See quote for my gamertag, steam name, and youtube :D
You need to shoot it several times until it disappears for good. Shoot, walk away for ten seconds, go back. You have to "kill" it and it took me about ~20 shots to do that. Could be more or less shots depending on your stats.