The Cockamouse was a hybrid creature that Marshall and Lily found in The Apartment in Matchmaker. It was first spotted by Marshall and Lily, who quickly raced downstairs to MacLaren's to tell the rest of the gang about it. However, none of them believed it, especially the highly-skeptical Robin. Marshall manages to trap the Cockamouse, and takes it to a biologist at the Columbia Biology Department for identification. However, the trap was empty. Lily and Robin later found it in the apartment, causing Robin to accept its existence. Marshall quickly grabs it and throws it out the window. Thinking it dead, they are shocked to learn the Cockamouse can fly, quickly shutting the window before it could fly back inside.