From the Super Mario Wiki
Bouncies are pink bubble-like enemies in Yoshi's Island DS. If one is touched, the character who touched it will bounce back a few feet. Sometimes, these circular creatures can help the Yoshis by making them bounce a few feet in the air and letting them reach high places. Bouncies can also help Yoshi speed past fast-moving crushing walls by bouncing him at a high speed. The boss Six-Face Sal is similar to them, except the boss can't be bounced on. They are prominently featured in the minigame Bouncy Maze.
Bouncies first appear in Level 2-5 of Yoshi's Island DS; this level was called "Boing!".