Enormous beings in whose intestinal tracts tummy achers originally lived as parasites.
The system's description of Tummy Acher pets, read when Carl examined Sir Rendlegore, explains the species' unflattering alternate names — Belly Achers, Colon Worms — by noting they were originally parasites living in the intestinal tracts of Jotan-class Titans.
The reference establishes the scale involved: a creature that begins fist-sized and grows large enough to swallow just about anything was, in its natural state, small enough to be a gut parasite in something far bigger.
Titanic beings large enough to host fully-grown tummy achers as intestinal parasites.
Original host species of the tummy achers.