From the Super Mario Wiki
A Barrel Shield is a shield that protects Dixie and Kiddy from acorns thrown by Minkeys. They appear in the hollow-tree levels of Donkey Kong Country 3 and Donkey Kong Land III, where they respectively debut in Barrel Shield Bust-Up and Minky Mischief.
Barrel Shields are seen with most groups of Minkeys. They either remain in one position or move with the rope. The moving ones repeatedly move up and down, starting with the first Minkey and ending with the last one. The Kongs must keep up with the Barrel Shield. Some move faster than others. In some cases, it is mandatory for the Kongs to hide behind a Barrel Shield, as some segments have too many Minkeys.