POSTNATAL TAILS. “All chordates possess, again at some stage of the life cycle, a postanal tail. Because the tail is retained in many ‘adult’ (i.e., sexually mature) chordates—and in nearly all adult vertebrates, with the major exception of apes and frogs—this can often be viewed as a true evolutionary vestige when the tail is partly or wholly non-functional in adults.”
https://evolution-outreach.springeropen.com/articles/10.1186/s12052-014-0012-5