A tail made completely of water. Sometimes this can be portrayed as looking like a ‘wave’ or ‘waterfall’.
Dog: https://waterdogworld.com/character/GST-5881
Wave shaped tail: https://waterdogworld.com/character/MYO-6536
Cat: https://waterdogworld.com/character/MOD-5050
Big cat: https://waterdogworld.com/character/BRD-2716