Mullfish

Mullfish are a type of fish that swim in a Mullhorn's stomach, entering into a symbiotic relationship with the huge herbivores. These clever creatures have adapted to survive in the Mullhorn's acidic bellies without being digested themselves. They enjoy a diet of whatever edibles a Mullhorn indiscriminately swallows as it scoops up whole mounds of grass, which includes various types of bugs and even small mammals such field mice or careless birds. In this way, the Mullhorn provides protection and nourishment for the Mullfish, while the Mullfish snacks on potential hazardous intruders and secretes an medicinal excretion that extends the beast's lifespan.
The Mullfish spends most of its life in a single Mullhorn's belly. As en egg, it is laid in a patch of grass in the path of a trekking Mullhorn. The egg can lay dormant for months even years until it is swallowed by a Mullhorn. Once inside the acidic belly, it will start absorbing nutrients, hatching inside. Near the end of its life cycle, it will lay its eggs inside of the Mullhorn, which excretes them in its wake, leaving them be eaten and incubated by another Mullhorn. Mullfish reproduce asexually, without the need of a partner to fertilize the egg.
The Mullfish's medicinal effects are well known by the shamans of the steppes, who use it as a panacea to treat all sorts of ailments. Recently, these boons have also been discovered by other Emdarians, leading to a rush on Mullfish Eggs. The nomads of the steppes warn against this reckless poaching, since the health of a Mullhorn herd depends on the Mullfish that keep their bellies clean from parasites and without this precarious balance the sacred Mullhorn will become prey to the tiniest predators.  This is why, in recent years, hunting for wild Mullfish has been outlawed, leading alchemists from House Reimdal to attempt to grow Mullfish outside of a Mullhorn's belly. They have had some succes raising Mullfish in acidic solutions, though the medicine prepared from these draughts have been nowhere near as potent as their natural counterparts.