I don’t think it’s important that they are that much of a challenge outright - the challenge should come from it being a surprise.
Rather than have it happen immediately, have it take a couple turns. So the player shoots the turkey hulk, breathes a sigh of release, heads over to butcher the great smelling corpse… and then has a brute RIGHT in his face roaring at him.
The chicken could be a “last hurrah”, and it’s default behaviour could even just outright be “flee! flee!” (It is a chicken after all)