This is because all the previous evolutions are based on the special based types in Generation I to III. Fire, Water, Grass, Electricity, Psychic, Dark and, Ice. The only one missing is Dragon. Users have frequently posted that that the Sylveon is derived from the word 'sylph' and, it regards a fairy, I'm not to familiar with the theory. Fairies are mythical beings as are Dragons. Altaria is a pretty stupid looking Dragon.