Any natural projection or excrescence from an animal,
   resembling or thought to resemble a horn in substance or form; esp.:
   (a) A projection from the beak of a bird, as in the hornbill. (b) A
   tuft of feathers on the head of a bird, as in the horned owl. (c) A
   hornlike projection from the head or thorax of an insect, or the head
   of a reptile, or fish. (d) A sharp spine in front of the fins of a
   fish, as in the horned pout.