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:

 

	Clever Dungeon North		Pack Dungeon South 2
	Devourer Dungeon North-East		Skylord Dungeon West
	Hunter Dungeon South		Massive Dungeon South-East

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.