undertail

adj

1. (not-comparable)Beneath a tail.
2. (not-comparable)On the underside of the tail.

noun

1. The ventral part of the tail of a bird.
2. The underside of a tail.

   “The whiptail wallaby is a slender, long-tailed, long-limbed macropod. […] its belly and the base of its undertail are white; the rest of its tail, feet and paws are dark.”