The Flyin' Wotsit Fingy is a small green bat creature that appears in The Trap Door episode of the same name.
During the episode, Berk is trying to take a photograph of his friends Boni and Drutt when suddenly this little bat - which Berk unceremoniously refers to as a "flyin' wotsit fingy" - bursts out of the trap door. The creature starts fluttering around the castle cellar and Berk chases after it, trying to keep it from going upstairs. Apparently, The Thing Upstairs is allergic to flying things as Berk states they bring him out in 'orrible purple pimples, which Berk will have to pop. As Berk chases the flying fingy around the cellar, he crashes into his camera's tripod and the camera goes off, taking a rather artistic-looking shot. As Berk and co. admire the accidental photo, they wonder where the flying fingy disappeared to. They quickly find out when Him Upstairs loudly complains that there's a flying thing in his bedroom.