[ddbach]

Normal tissue in an abnormal location = hamartoma

These may cause little to no problem or cause significant dysfunction based on the type of tissue it is and where it is misplaced. It sounds as though it may be a volumetric issue so perhaps the mass has slowly enlarged during his life. If he is otherwise healthy I would expect him to do very well. Would they remove it using a cystoscope?

Developmentally the bladder forms by closing along the edges of an invagination rather than forming a lumen in a solid mass. This is the stage where some cells that were "programmed" to be skin were in the wrong place. Considering how many changes occur during the development of a living organism it is amazing how we, our pets and other things turn out easily recognisable.

Disclaimer: not a vet