Here is my entry for the under the sea skin contest, based on capricorn, the sea-goat/zodiac sign. Constructive criticism is appreciated (I'm still relatively new to making skins), and be sure to diamond if you like it.
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Background Info:
(added at the advice of silverfox1234)
Capricorn is, in mythology, a goat with a tail. One myth about him is that he was originally the goat man Pan, who was chased by Typhon, the father of all monsters. He jumped into the Nile to get away, but when he left the parts that were submerged turned into a fish tail, while the part that stayed above water remained the same.

In my version he was originally just any old goat. This all changed one day when he was walking along and decided to get a drink. He was too far away from his regular pond, and he noticed a river at the bottom of the hill. What he did not notice was the waste facility across the way. As he got the drink he felt sick and fell in, already weakened too much to swim. However, he did not drown, instead he found he could breath the water, and was carried out to sea by the current. Over time he started to turn blue, and his back half mutated into a fish tail. As time went on he also developed sentience, and became fairly intelligent. He got the name Capricorn when some tourists came by and saw him, calling him by the aforementioned name. It stuck, and now people all over come in search of him, hoping to find this sea-goat.