undertail

1. not comparableBeneath a tail.
2. not comparableOn the underside of the tail.

   Synonyms: subcaudal

plural undertails

1. biologyThe 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.

   2005, Reader's Digest, Encyclopedia of Australian Wildlife