Rita Skeeter is the Animagus who becomes a beetle in J.K. Rowling’s Harry Potter series. Her unregistered Animagus form allows her to spy on conversations and gather material for sensational articles.
The revelation appears in Harry Potter and the Goblet of Fire. Hermione Granger discovers Skeeter in beetle form and traps her in a jar, later using the evidence to force the journalist to stop publishing hostile stories for a period. The detail explains how Skeeter obtains information that should be inaccessible to a normal reporter.
This fact is often confused with other Animagi in the series. Peter Pettigrew transforms into a rat, Sirius Black transforms into a large black dog, and Minerva McGonagall transforms into a tabby cat. Unlike those registered or openly known cases, Skeeter’s transformation is illegal because she never registered as an Animagus.