The Gelatinous Non-Cube was an overgrown Blobikins that hid in many rooms of the Temple of a Thousand Tombs. Coming across the Gelatinous Non-Cube would result in the Neopian's inability to enter the temple for a long while, basically adding a nuisance to the puzzle plot.
It also made a reappearance in the Altador Plot inside the Restive Tomb, where it appears to be mummified - and subsequently has moved into the vaults below Neovia.
Researchers have suggested that the non-cube is simply a normal Blobkins that has just eaten an awful lot, meaning that there may be many of them in Neopia.