From the Super Mario Wiki
The shorter, purple variant of Dragoneels appear in New Super Mario Bros. U and its Nintendo Switch port. Only two purple Dragoneels appear in a small section with Coins and a Star Coin of the Sparkling Waters level, Dragoneel's Undersea Grotto. They have the same strategies that Dragoneels have; they follow the player(s) while they must avoid them. However, they make tighter turns, and are significantly slower. Throwing fireballs at their heads will make them move gradually slower. They are one of the few enemies that do not reappear in New Super Luigi U.