They live in the old railway cave which is located in the fertile outside of the portal, you can’t bring any Dinos with you so bring a sword, a shotgun with bullets, zip lines, climbing picks and/or motorized zip line attachment and armor with an extra set. Once you find one they are tamed with the black mushrooms, don’t turn them on in the cave though because that only attracts the ravagers raptors and seekers. And, while you’re in the cave why not pick up the artifact? Also karkinos spawn in the cave so be prepared for those.
Use z seeds to tame for best leveling boost. Ascerbic mushrooms take longer and lower taming boost (but are faster to farm, ok for low level nightlights for base).
Note: they stay still for the most part when mating, but have noticed them scattering about occasionally so keep them in a secure pen.