This article is about toys that have lots of kibble. For toys from a long time ago that still haven't been sold, see Shelfwarmer.
A shellformer is a fandom term for a Transformer toy that transforms by "unpeeling" pieces of one form to reveal the other form inside. Literally, one mode (almost always the alternate mode) is a "shell" that closes completely (or close enough) around the robot parts. This term is often used derisively.
Note:DON'T confuse the shellformers with the Pretenders.