Animal!
Horzine was already trying to make supersoldiers for the government - why not engineered attack dogs? I'm thinking of a hairless wolf-like specimen with overdeveloped jaw musculature and a long tentacle-like tail. They'd be as fast as, or faster than a raging fleshpound, but have health roughly equivalent to a gorefast. They could have two attacks: a pounce that does minor damage similar to a crawler, and a bite to the leg that immobilizes the player for a few seconds, even if you are a berserker.
They would offer an immediate priority threat that is easily overcome but a real problem if you don't kill them on sight, a little "rush" for the players if you will. Maybe spawn them in packs of 3-6, and give them an AI that causes them to try and flank the player like the stalkers do.