Boss enemies are not running around like the other dinosaurs in Ark. These must be called by using an artifact. These arifacts can randomly be found in dungeons in a loot crystal. At the moment 6 artifacts are known:
Dungeon South 2
A list of the dungeons including a tiny tutorial can be found on our page: Dungeons and Caves
If you’ve got all 6 artifacts, you’ll have to insert them in an obelisk to summon the broodmother lysrix.