Caragors are the large cat like beasts you will encounter around the open world; their cages are approximately 6x6x6, metal & often near enemy encampments. The cages have one side that, when in the wraith world, shows a lock & chain, use your bow to shoot this side 1-2 times, depending on bow strength, to break it open. The cages can also be found embedded in certain stronghold walls.
Once opened the caragor inside will attack anyone within range, including you; as such it is advisable to use an ability point to unlock the ability to tame one, you can do this very early on as it is a level 1 ability.
The achievement will unlock in-game when the final one is released. If you are having any trouble finding them, story mission three or so (there are multiple paths), involves killing a captain who is afraid of caragors, this mission includes multiple cages along the path!