The junkyard dog appears to be a Rottweiler which can be bought from Gene, their current master. They were bred by and raised by Gene, and is potentially for sale to someone who he deems acceptable. Gene is extremely loath to sell him but if the Sole Survivor can pass a Charisma check of six or higher he can be persuaded to, painfully, let him go for 250 caps.
If bought, the junkyard dog can be sent to any workshop settlement that is under the population cap, and will count for five points of defense. They will also go into a doghouse to rest just like Dogmeat.
If the dog enters a settlement in workshop mode, they can be moved to the current or another settlement without paying for them, the dog will add 5 defense and occupy a doghouse. Passing the Charisma check with Gene may be necessary first.
When looking to find the junkyard dog at the settlement selected when purchased, the dog will be found at any available doghouse. One can remove all existing ones then place a new one where the player character prefers her to be located.
In the workshop interface the Sole Survivor can select/move the doghouse and they will follow around while carrying it.
Unlike with Dogmeat or settlers the Sole Survivor cannot trade with the junkyard dog. However, items can be equipped by reverse pickpocketing them to equip dog apparel.
It is possible to encounter Gene again after purchasing a first junkyard dog. If this occurs a Charisma check is not required to purchase another dog, nor is the conversation as long as the Sole Survivor states that they would like to purchase another dog.
The junkyard dog is the only character in the vanilla game with BonusHappiness which counts towards increasing settlement Happiness. The Wasteland Workshop add-on allows for acquisition of cats and gorillas that provide the same benefit. Cats do not contribute to defense rating or actual defense, gorillas do.
The junkyard dog's equipment may randomly change over time between the various items that Dogmeat can equip.
Xbox One If purchasing multiple junkyard dogs from Gene it can change their model, like fur color change.[verified]
Bug fix: reload a previous save before purchasing if possible then reload present save.
PC If the Sole Survivor pickpockets a companion near the junkyard dog, the dog and the other settlers will begin attacking the Survivor, and they will not become docile even after a 24 hour waiting period, unlike the settlers do. This can not be reset using console commands, they will always be hostile following this event. [verified]