Way too late into development to be adding crap like that.
The word ‘wildlife’ makes me think of Xen as some kind of alien jungle.
I always looked at Xen as some kind of cluster of floating rocks with a lot of fungi and such on them.
Xen is not supposed to be inhabited with anything naturally.
I always was giving the impression that it was the refuge for alien races running from the Combine – so a mix of species makes sense.
That’s is why the grunts appear to be some kind of makeshift militia for the sector.
The Devs are not gonna add creatures that were not in the original. Especially not this late in development.