Sprite Darter Egg is used to summon Sprite Darter Hatchling, a faerie dragon companion.
This companion is a quest reward from A  ω τ ϖ. However, the quest chain leading up to it is rather long and convoluted. A guide to the chain can be found at the Freedom for All Creatures quest chain page. As patch 3.1.0, Horde members can obtain this pet too. It rarely drops from Sprite Darters in Feralas.