Eyeball is a special skin that was included as part of the Legendary Pack in the middle of April 2017. Legendary was one of the three Packs available as part of the occasional Booster Pack offer.
Eyeball could be purchased for $19.99 as part of the Legendary Pack. This Pack also included 50000 coins, a Mass Boost (3X, 24H) and a XP Boost (3X, 24H). Eyeball was not available for individual purchase and may not be available for purchase in the future.