From SKYbrary Wiki
A horizontal, tail-shaped cloud (not a funnel) at low levels extending from the main precipitation region of a supercell Cumulonimbus (Cb) to the murus (wall cloud). It is typically attached to the wall cloud, and the bases of both are typically at the same height. This makes the Cauda look a bit like a dinosaur's tail trailing behind the storm and thus it is commonly known as a 'tail cloud'. Caused by air flowing into the storm, its tube-like shape can sometimes be mistaken for a tornado. It is a supplementary feature of cloud.