Development & Concepts

In the earliest stages of StarCraft II, hybrid were considered as a fourth race for the game. During development, the Xel'naga were considered as a possible fourth race, but as lore was developed, it was deemed that the Xel'naga were too powerful to be playable. The hybrid ended up becoming "Xel'naga proxies" within the campaign. 

Blizzard went through thousands of hybrid designs over the course of StarCraft II's development. Out of the thousands of forms pitched, only a few were locked down. The cut designs were "aquatic and graceful," intended to convey an alien look that was more advanced than the Protoss and Aerg. 

Cut lines for Heart of the Swarm indicate that hybrids would have been encountered outside Skygeirr in a pre-release version of the game.