It appears to be based on legend of the nekomata, also known as bake-neko. Nekomata are mysterious two-tailed goblin cats. Simultaneously, Espeon also appeares to be based on an Egyptian goddess of the sun Bast, who was typically depicted with the head of a cat.
Espeon's name is derived from ESP, or extra-sensory perception, a branch of psychic abilities. Eon was the proto-type for Eevee, Espeon's pre-evolved form. Espeon's Japanese name is probably derived from the fact that it's forked tail looks like a capital letter "F".