Sneetches are self propelled flying exploding objects used by the Wolves in Wolves of the Calla, as well as the Crimson King from the balcony of the Dark Tower he has become trapped on. They appear to be able to seek targets, and can have a timer set on them to countdown, both methods are used in Wolves of the Calla and The Dark Tower.
Intertextual References
They bear similarity to the Golden Snitch from the Harry Potter series' game of Quidditch (in being small flying spheres, they are not weapons in that series), and are generally seen as a nod to Rowling, the author, whom King has been a supporter of.
They are also a reference to a race of creatures in a book by Dr. Seuss.