If you're talking about the wolf, you get it after defeating Luna Nightshade.
And if you talk about the shark, you get it after doing the 3 maris missions
Viewing post in How do you get the companion
trust me, luna nightshade and her clan are no joke, every dog u battle against not only dealing massive damage, they can poison ya as well. Make sure u use nightshade poison dropped from them and bleeding effect against luna nightshade, both poison and bleed can break her down in both of her phases, plan carefully.