Cow31337Killer is a spoof of an adventurer occurring during the Animal Magnetism quest. He kills an undead cow, and attempts to kill an undead chicken during a cutscene. He is shooed away and can't kill the chicken. He is combat level 123, and wears rune armour with a Dharok's helmet and a wields a Greataxe. His name appears to be "Cow Elite Killer" in Leetspeak.
Previously, this NPC's name was Cow1337Killr