Cupcake is a fictional animal character in the Hey Arnold! TV Series.
He is a chocolate-colored blue-eyed kitten found by Arnold in a dumpster and adopted by Harold. When Arnold found out that the kitten was lost by Mrs. Ryle (who calls him Thurston), Harold refused to give him back. Returning Cupcake/Thurston to his owner required help of police, animal control service, and Arnold.
Harold named the cat 'Cupcake' because he looked like a "chocolate cupcake with a cream filling"... (See Hostess Cupcakes)