From the Super Mario Wiki
The shorter, purple variant of Dragoneels appear only in New Super Mario Bros. U. 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 of the Dragoneel; they follow Mario, while the hero 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.