"A mature neothelid is a gigantic worm scores of feet long and weighing tens of thousands of pounds". - d20srd.org

A Neolithid is the product of an Illithid reproductive cycle gone wrong. The tadpoles kept in the Elder Brain tank are constantly feeding on each other, to produce the strongest ones by the time they are ready to be implanted into another lifeforms brain. If one gets too big, the Illithids will see it as a potential threat and kill it. A Neolithid is one of those, but it grows too big for the Illithid caretakers to kill it in time. By the time they check on the tadpole pool, the Neolithid has most likely escaped the pool, being several feet long. From there, It grows bigger and bigger, and feeds on the brains of other creatures.

The base is a 4-inch "Gargantuan" size. The design is intended for D&D but you may use it for other RPGs.