Nothin' But a Hound Dog

 is a quest in Fallout: New Vegas.

Quick walkthrough

 * Talk to Julie Farkas about what can be done to help Rex.
 * Return to the King with the news that Rex can be healed.
 * Take Rex to Doctor Henry in Jacobstown to learn how he can be fixed.
 * Acquire a new brain for Rex from one of Old Lady Gibson's dogs.
 * Rey's Brain increase Rex's attack.
 * Violetta's Brain increases Rex's movement speed.
 * (Optional) Search for an exceptional canine brain among the Fiends.
 * Lupa's Brain increase Rex's DT.
 * (Optional) Search for an exceptional canine brain among Caesar's Legion.

Detailed Walkthrough
Talk to King in Freeside. Ask him about Rex and he will tell you that something is wrong with him. He will then ask you to go to Julie Farkas. She will tell you that there is a man, Dr. Henry, in Jacobstown who can help. You must then tell King that Rex can be fixed, having him allow you to take Rex to Dr. Henry. He will tell you to to find at least one dog brain to fix Rex. The first brain you need is at Old Lady Gibson's scrapyard, just north of Novac. She will ask you for 700 caps, if your barter skill is at 70, it will be less. The second brain is at a Fiend camp near Vault 3, there are two sections to the camp, one is a type of kitchen with Cook-Cook as the leader, in the other section of the camp is a structure built from ruined buses, this section is populated by Fiend Guard Dogs, Violetta, a boss Guard Dog and a hound master called Violet, you can acquire Violet's head after killing her and you can acquire Violetta's brain after killing her. Choosing to implant Rey's brain will give Rex a perk to increase attack damage, Choosing to implant Violetta's brain will give Rex a perk to increase speed.

Behind the Scenes
This quest is a reference to the Elvis song of the same name.

-Hound Dog is an original song by Big Mama Thornton and Elvis Presley was among one of many who covered it.