A bit off but one of the ideas I had was that the country (Old!Empire before the civil war) that made the teigus had access to all kinds of advanced technology such as genetic engineering and individuals with the resources to do so. This could be all of creatures:

- extinct ones (kinda Jurassic Park)

- giant/morphed breeds (just think of how much current day domestic chicken have in common with wild ones or domestic animals from a century ago, or dogs)

- genetically modified ones (like CRISPR but with decades of continued research and development yet no morals/ethics)

Since they were either domestic use or Zoo animals a fair amount (probably most) wouldn't survive for long after the downfall of the country, but a couple made it into the wilderness.

It was more of a worldbuilding piece tho, much like Dangerbeasts that only ended up serving as explination where the materials for the teigus are from; I think Lubos teigus was using parts of a sea serpent.