Eelektross is the only Pokémon Ben is known to have on hand. It was used in the first round of the Higaki Conference against Kotetsu's Ferrothorn. It was defeated in BW104 when Ferrothorn launched a Pin Missile attack into its mouth.
IF a new type is introduced then I think it would be light.
However as for now it is looking like a flying type as it has been put with Jolteon, and eeveelutions are always put with their (dis)advantage types such as Glaceon and Flareon.
So it could either be FLYING or GROUND.
Its design is more on flying than ground.
AND if a new type is introduced then that being light. Then I think Xerneas would be a light type and Yveltal will be dark.