From the Super Mario Wiki
Ultra Char-chars are fire enemies that appear in Donkey Kong Country Returns and Donkey Kong Country Returns 3D. They are a variant of another fire enemy, Char-chars. They are much bigger in size than Char-chars. Ultra Char-chars appear in the Volcano level Furious Fire, as well as the remake-exclusive Cloud level Lavawheel Volcano. In this level, they roam around, trying to harm Donkey Kong. Ultra Char-chars cannot be defeated by any attack method, including jumping and throwing barrels. They are red ball-like enemies with a "tail" of fire. They slightly resemble comets.