Are you sure it doesn't say "It's the barb from a slasher tail"? "the barb" implies that each slasher tail has only one barb. Therefore, even though it's not been mentioned before, we do in a sense know "which one it is", as there is only one (per tail).
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GPY"a barb" could be used whether the tail has several barbs or just one.Oh that's interesting. Let me ask this
Russkiy BearOh that's interesting. Let me ask thisThe show that was shown on ABC yesterday at 7 pm was greatI can still say a show...?"A show" would be rather unusual in that sentence.