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I got this idea while browsing the "New Zombie" poll thread.
Think about it, it's obvious that the headcrab's physical structure is designed so that the headcrab is particulary suited for human hosts (heads). They got a big round 'mouth', and are fans of the surprise in-the-back-from-the-corner attacks. I really doubt they could leap on any other non-human creature (garg, vort or antlion).
And since the headcrabs originate from Xen (as long as i know), how did they managed to survive out there, before the resonance cascade brought them the new type of 'steak' to feast on :E
Or maybe they are designed on earth to be used as a biological weapon (as the combine eventually uses them)?