Swish-tail was a pony belonging to Meriadoc Brandybuck.
Swish-tail was one of five ponies that Merry had ready for the Hobbits' journey at Crickhollow. At the home of Tom Bombadil, the ponies became friends with Fatty Lumpkin, a pony of Tom's. At the Barrow-downs they became so frightened that they ran off to find Fatty. Tom called them all back with the names he had given them: Sharp-ears, Bumpkin, Wise-nose, Swish-tail, and White-socks. Eventually they all returned with all the bagge, and went back on their journey with the Hobbits.
During their stay at the Prancing Pony the Hobbits' ponies were let out to delay their departure from Bree. They ran out in the Downs to find Fatty. They stayed with Tom for a while, until he had heard news of the events at Bree. He sent the ponies to Barliman Butterbur, and they stayed there working for Barliman, they were taken well cared for by Bob.