From the Super Mario Wiki
Mini Ninjis are Ninji toys in Mario vs. Donkey Kong. They act just like Ninjis and look like them with a keywind on their back, like all the rest of the Mini toys. They jump up and down, which can give Mario a boost at getting to high cliffs. They have few appearances in the game, first seen in Fire Mountain. It is possible they were made by the Mario Toy Company.