The Xel'naga Destroyer was once the only type of Hybrid unit in StarCraft II. This was later balanced through the Zerg-based Hybrid Reaver.

StarCraft II beta files list it as a "Xel'naga Destroyer".

Xel'naga Destroyer was later replaced with the now known Hybrid Destroyer

The Hybrid Mastermind, also known as the Hybrid General, is an unfinished unit in the map editor for StarCraft II: Legacy of the Void. 

The mastermind uses the same model and sounds as the hybrid destroyer, scaled up. The mastermind has two abilities, one which is similar to Nova's Domination ability, but the mastermind can take control of multiple units at once. The ability is unfinished and not named. The second ability is the same Phase Shift as the hybrid nemesis, phasing a target unit out of existence for a period of time. 

Xel'naga Keepers (also spelled "Zelnaga Keepers") were a scrapped unit concept made for a Xel'naga armored footsoldier in StarCraft II. Keepers would have a hard armored exterior, which after being destroyed would reveal a powerful psionic being within. They were designed to ensure the Xel'naga's return. 

Gameplay concepts included a timed life, high power level, shields that worked like Protoss Plasma Shields, and a life drain ability. However, designers found the Xel'naga themselves were too powerful to create a faction around, and the keeper bore too much resemblance to the Protoss. Thus, the concept was scrapped. 

Hybrid Dominators originally possessed the ability to merge with one another. The original mission mechanics involved them creating a psionic vortex that would destroy Ketill if three merged. 

Originally the protoss-based Hybrid Destroyer was the only type of Hybrid unit in StarCraft II. The idea of a Zerg-based hybrid had been pitched by Samwise Didier in concept art, (labelled as a "high breed")  but was effectively abandoned for a year. Eventually the idea of a zerg-based hybrid was pitched within the design team and Didier's concept was resurrected as the Hybrid Viper. In the final game it was known as the Hybrid Reaver mostly the same, but unlike the Viper, it had tentacles coming off its back, that aren't visible on lower graphic settings (the player must set "effects" to medium or above)