Sylveon is the eighth possible evolution form for Eevee, which originally appeared in the first generation of “Pokémon” video games. Since Sylveon has been revealed, not much is known about the Pokémon's type, only a few clues to ignite great speculation across the Pokémon community.
Pokémon Smash gave some clues about Sylveon. They "tested which types Sylveon fits" in a small segment, by dressing people up in different Eeveelution costumes, telling them to perform attacks on Sylveon. Their findings were:
Sylveon is not a Fighting type.
Next, they tested to see if Sylveon was strong against Ice – PASS!
Sylveon was strong against Ice, so they tested if it was a Steel type by using Poison type attacks on it – NOT RESISTANT TO POISON!
What we do know:
The only four types that are super effective against the Ice-type are: Fire, Fighting, Steel, and Rock.
Flareon is already the Fire-type Eeveelution, and it has already been said that Sylveon is not a Fighting-type, thus leaving it to be either Rock-type or Steel-type given the types that are currently available. However, Steel was also ruled out, leaving Rock-type as the only possible current type that Sylveon could be. But, Sylveon does not appear to be a Rock type, as each Eeveelution's type is easily identifiable judged on their appearance. This leaves us to believe that Sylveon is most definitely a new type.
Could it be a Fairy type?
Sylveon's name in some other languages is:
French – Nymphali – nymph (a type of Fairy)
German – Feelinara – fee (German for Fairy)
The starting name of each Eeveelution usually directly relates to the type it shall be, for example:
134 – Vaporeon – vapor (water mist)
135 – Jolteon – jolt (burst of electricity)
136 – Flareon – flare (burst of fire)
196 – Espeon – ESP (Extra Sensory Perception: psychic power)
197 – Umbreon – umbra (darkest part of a shadow)
470 – Leafeon – leaf (plant foliage)
471 – Glaceon – glacier (a mass of...