From the Super Mario Wiki
Bouncies are pink bubble-like enemies in Yoshi's Island DS. If one is touched, the character who touches it will bounce back a few feet. Thereby, these creatures can help the Yoshis to reach high places. Bouncies can also help Yoshi speed past fast-moving crushing walls by bouncing him at a high speed. They are prominently featured in the minigame Bouncy Maze.
Bouncies first appear in Level 2-5 of Yoshi's Island DS, called "Boing!".