The Goldfish of the Fish Tank
Everything the cat did screamed normalcy. From the way it walked, carried itself, harassed others, etc. It was no different to a cat from before the apocalypse.
But, two factors set it apart, something it had glossed over. First was the fact that it was clean, too clean even though it had just walked through a corridor filled with blood.
Even though a cat had the habit of keeping itself clean, it wasn't to this extent that no trace of scent of the blood would linger on it. The level had clearly traversed into the abnormal territory.
And, the second oddity was the horn that was growing on its forehead, a clear indication that it was a mutated beast. Moreover, Compass Carburettor noticed that a second horn had begun to grow too.
Though, it had barely protruded out of the cat's forehead, "But, within a week, it would become a two-horned cat."