Compsognathus

Compsognathus, trapped in caves, rode out the extinction of most other dinosaurs, and evolved into the Zerg. Compsognathus could have eaten, and mated with, and fought with essentially only instances of its own species for millions of years before emerging back into the open world.

Literally, potentially a source of ancestry, explaining the extreme degrees of bizarre behaviors deliberately undertaken and maintained by the Celtic aborigines (up until their genocide).