If you look at some of the older Krogan head plates, many are very distinctive with protruding, edged ridges that sweep rearwards. It's not difficult to believe that during the evolution of the species, there were certain genetic traits that produced head plates with forward sweeping ridges that resembled horns. Maybe those genetic traits are gone in the timeframe that ME1-3 takes place, maybe certain clans still had this genetic trait. Perhaps it is still there, rare and with a stigma attached so those that do have a horn grind it down. Perhaps it is even a body modification, an artificial horn anchored to the headplate to serve as an addition melee weapon. Or maybe the genophage cure has brought forth some recessive genes as a side effect. All are within the realm of possibility, as completely fictional chitin covered space frogs are concerned anyway.
As for combat, just because a Krogan has a horn doesn't mean it can't headbutt, instead spearing or impaling enemies.
Regardless, the new concept art looks awesome and I'm extremely excited about the official announcement of a new Mass Effect game. I cannot wait for this game to launch because I've bored to tears and barely play video games at all anymore. Mass Effect has made all other franchises seem lackluster to me.