From Monkey Island wiki
Miss Prettywhiskers is Hemlock McGee's pet cat. In "The Trial and Execution of Guybrush Threepwood", McGee accuses Guybrush of paralysing her in fear during his first, blindfolded visit to Club 41 three episodes before. Miss Prettywhiskers remains immobile the entire time except for her eyes.
Guybrush sneaks some iron shavings into the cat's food and then pulls out Jacques, the electromagnetic monkey, in court; Miss Prettywhiskers is drawn to Jacques and seems to be attacking him, removing the blame from Guybrush.
Miss Prettywhiskers is called by three different insulting names: first by "Miss Grungywhiskers", then by "Miss Trashywhiskers", and finally by "Miss Barfywhiskers"; each time Guybrush calls her by the insulting name, Hemlock gets into a Poxed rage and insists on calling her "Miss Prettywhiskers".