Gnaw

Gnaw, known as Growl in the PAL version, is a large, imposing creature that is purple in color, with blue spots covering its body, several rows of sharp teeth, and some retractable spikes on its head (which will not actually deal any damage to Mario or Peach if they jump on them). Gnaw is also immune to any form of physical harm, making it unable to be defeated.

Gnaw is Merlee's favorite pet, which she also uses to guard her mansion. In Super Paper Mario, when Mimi took over Merlee's Mansion she chained the Gnaw to a door, making it incapable of attacking her. If Mario or Princess Peach were to approach Gnaw, it would try to grab them in its mouth; if Gnaw succeeded, it would give its catch to some Gnips, who would proceed to throw them out of Merlee's Mansion. The reason it did this was to ensure that Mario and Peach did not get hurt by Mimi, as it knew that Mimi was evil, whereas the heroes were fooled into thinking she was Merlee's maid.

If Mario or Princess Peach managed to obtain a hidden key in Merlee's Mansion, they would need to sneak by Gnaw and use it on a nearby door. Unlocking the door also frees Gnaw, who proceeds to chase a nearby Mimi all throughout Merlee's Mansion. It presumably still resides in the mansion after the game, no longer having any problem with Mimi as she had reformed and been made the mansion's rightful owner by Merlee.

Gnaw's Catch Card can be purchased from Flimm for 113 coins, though he doesn't always have it in stock.

Tattle
"This is Gnaw, Merlee's favorite pet. It's certainly the biggest of of the bunch...  But some mean person tied it down...  And I think it's worried that this mean person wants to hurt you...  Good Gnaw!"

Trivia

 * Gnaw is similar to a Chain Chomp in that they are both dog-like creatures with sharp teeth. In the first Paper Mario, the Chapter 2 boss was chased away by Chain Chomps, and Gnaw chasing Mimi (another Chapter 2 boss) may be a reference to this.