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Sylveon's Japanese name is 'Nymphia'.
Nymphia is derived from the word Nymph.
There are 5 different types of nymphs, Celestial Nymphs, Water Nymphs, Land Nymphs, Plant Nymphs and Underworld Nymphs.
Water would be Vaporeon. Plant would be Leafeon. I'd guess Underworld to be Umbreon.
Which leaves Ground and Flying types.