Well, from what we heard from Walter, the David line was unique to androids, and I'd say David's personal experiences make him unique to the David line. Unless of course, one of his earth-bound counterparts worked at a landfill and bioengineered/controlled a larger and more aggressive form of rodent. With he and his creatures bent on taking over the city. Perhaps all the David's went nuts like our David. Maybe somewhere out there, a David is sitting inside a maintenance closet with an old printer, some playdoh, and rubber ducky on the bench saying, "Big things have small beginnings".