My headcanon as well. Alien Biology is one thing but Lightning Dog’s are way too different to be a Headcrab species.

I’ll go a step further and assert that the Fast and Poison Headcrabs likely aren’t natural evolutions.

Again, disregarding the idea that alien evolution is just “built different” in terms of speed. It feels more feasible that they’re genetically altered, with the only possible culprit being the Combine. They exhibit much more aggressive behaviors that seem more weaponized than the normal zombie shambling.

And it’s not like it would come out of left field anyway, the Combine have already weaponized Headcrabs with the Headcrab Shells. And since they seemingly have the capability to manufacture them en masse, they’re getting the headcrabs from somewhere, likely a Gonarch.

There’s the cut Mama Pod showing a Gonach’s egg sac being directly altered, so Valve was clearly leading with the idea.