The third pet to join his stable is BREGO.
Jaronn was out in the Plaguelands and ended up doing a quest on the Marris Stead, where the Blighthounds are. Inside the building, one was there but did not attack (it was bugged and caught in something). It would spin to follow and growl/bark, but did not actually do damage. Something about that just felt right and I ended up taming him.
The story is Jaronn found an injured mastiff amongst the others, not as infected or diseased as the others. Feeling sorry for the mastiff, he coaxed it to trust him and nursed Brego back to health.
