Splounder

The invincible Splounder first appears in Super Mario 3D World where it cannot be harmed at all...but can be jumped on by Plessie. In fact, Splounders appear only in levels where Plessie is featured.

If Plessie bumps into the Splounder enemy, there will be a momentary setback and Plessie will get pushed. However, jumping on this enemy will prove useful as you may be able to nab a collectible because of the extra bounce given to you by the Splounder.