A Slithershock Elver is an Aquatic Battle pet, which has a small chance to drop from Travler's Pet Supplies, the reward from the daily quest Critters of Draenor, which is given by Kura Thunderhoof and Erris the Collector, for Horde and Alliance players, respectively. Kura and Erris have a random chance each day to spawn near the menagerie in each garrison. If they are not there on a particular day, you can visit the garrison of another player who does have one of the traveling pet tamers, to complete the quest for that day.