Final Metamorphosis

Here's a weird theory: a third type of "hybrid".

Spoiler Alert!!!

If anyone has read Shadow Hunters, they would have noticed Ulrezaj's experiment. He made his minions capture Zerg, then he "drained" Protoss, seeming to add their energy to these Zerg. The resulting "enhanced Zerg" lay in strange vats in caverns under Aiur. They had psionic abiilties and looked a bit like Zerg (hard to tell, since they were sitting in inky water).

The drained Protoss were kind of like Dark Templar (that's another spoiler!) while the Protoss used in Duran's experiments were High Templar.

Ulrezaj was said to have a boss. What if it's Duran? If so, this could explain why the Hybrid army has more than one unit. There's Duran's "high" Hybrids and Ulrezaj's "dark" Hybrids, giving an in-lore reason for why there's more than one type of unit. (I can picture a desperate battle with the Protoss again Duran's Hybrids when Ulrezaj's cloaked Hybrids show up and pwn the Protoss.)

No, the Zerg absorbed Xel'Naga. They didn't infest them. The Zerg only infest/assimilate powerful warrior species. The Xel'Naga are probably wimps. The Overmind just wanted their knowledge.

Oh, Queen face: http://starcraft.wikia.com/wiki/Image:Queen_SC2_Art.png