It is said in Galar that the Weald only began to slumber when the Drums of Rillaboom fell silent. This should not, however, be mistaken for an analogue to Loudred: the lone ancient eyewitness to write about the topic described it as a low, rumbling tone, and parts written to depict Rillaboom in ancient musical notation were written as no louder than normal for the drums used to portray them.

Rillaboom do not awaken so much as animate, bringing inert forests to life the moment they begin to play. Yet this is not a harmless activity, for many explorers lost their lives seeking the supposed Rillaboom habitat, and we know from modern examples that beating their drums to slow down their foes is one of their favorite techniques in battle. Yet some Rillaboom are gentle; their forests close around intruders, but leave them a way out, or simply seal themselves up before any can enter.

Whenever a new forest grows, Galarians prove wary of entry, and are content to leave it to its own devices until it can be proven no Rillaboom lies within. This custom, which survived the last securely documented ancient Rillaboom sighting by centuries, has often been credited with leaving a great deal of Galar as wilderness. But it should not be overstated, for almost none of this wilderness is Grookey habitat.

Explorers and cryptozoologists long doubted the extinction of the wild Rillaboom, but it was not their efforts, but a trainer raising a domestic lab Thwackey, who first showed this pokemon to modern eyes. As the fearsome drumming ape of the forest, it was a disappointment, bereft of style and grace. But as a great discovery of the Internet age, Rillaboom became a popular character in memes, owing primarily to its humorous face.