This page of our guide to Animal Crossing New Horizons describes a cat named Rover.
Rover is a blue cat you met at the beginning of the game, during the character creation process (though you may not remember this). However, this is not the cat's only role.
During the holiday days in May (from 1st to 7th of May) you can come across a special island-maze. Tom Nook will give you a ticket. This island has a simple puzzle in which you have to pass through a maze. At the end of it you will meet Rover - he will reward you for your effort.