Rex Companion Guide: Nothin’ But a Hound Dog & Brain Choices

The King’s cyberdog

Rex belongs to The King in Freeside and is introduced as a beloved companion with a failing cybernetic brain. Helping him turns a local favor into a quest that reaches several parts of the Mojave.

Nothin’ But a Hound Dog

The quest sends the Courier to find a compatible replacement brain. Different donors produce different statistical benefits and small ending variations, so the decision can be made for role-play, combat value or both.

Search and Mark

Rex helps highlight useful containers and items while travelling, fitting his role as a cyberdog that can sniff out resources.

Why Rex is a useful archive character

Rex connects Freeside, The Kings and pre-War cybernetic technology without needing a large amount of dialogue. His quest is a compact example of how New Vegas uses companion mechanics to send players into unrelated communities.

Recruitment through the Kings

Rex belongs to the King in Freeside and is introduced through the Kings’ quest network rather than found wandering the desert. The cyberdog is suffering from a failing brain, which leads into “Nothin’ But a Hound Dog.” The quest sends the Courier looking for a replacement brain, turning a companion recruitment story into a choice about what qualities Rex will inherit.

Once Rex becomes a permanent companion, Search and Mark makes nearby containers, weapons and useful items easier to spot while exploring. The perk is less dramatic than a direct damage boost, but it suits scavenging-heavy play and makes dense interiors easier to read. Rex occupies the non-humanoid companion slot, so he can travel with Boone, Veronica, Arcade, Raul or another human companion.

The three brain choices

The replacement-brain objective offers multiple candidates, and the choice affects Rex’s resulting abilities. Rather than treating one option as universally “correct,” decide what you want from the cyberdog and how much the source of each brain matters to your Courier. The game turns a mechanical upgrade into a strange little identity question: Rex remains recognizably Rex even as an essential biological component comes from somewhere else.

That theme fits Fallout’s broader treatment of bodies, machines and continuity of self. Synths are more prominent in Fallout 4, but Rex and ED-E already ask the player to care about companions whose identity cannot be reduced to ordinary human biology. The archive links those characters because the mechanical question—repair or upgrade the machine—is never the whole story.

When Rex is most useful

Rex is a good exploration companion for players who enjoy looting and close-range support. He moves quickly and can engage enemies aggressively, which is helpful in ordinary combat but worth managing around neutral NPCs or fragile stealth plans. Search and Mark remains the main reason to bring him when you are clearing cluttered ruins and do not want to overlook containers.

Because his quest touches Freeside, the Kings and several potential brain donors, it is worth resolving before the final faction route if you want a complete companion ending. The quest does not decide Hoover Dam, but New Vegas’s ending slides remember personal outcomes precisely so these smaller stories do not vanish behind the battle.

What to watch while playing

Rex’s quest is unusually mechanical for a companion story because the replacement brain changes the cyberdog’s final statistical emphasis. Keep associated faction (The Kings) in view alongside companion perk (Search and Mark). Type adds the third piece: Cyberdog.

The three donor choices reward different priorities, so there is no single universally correct brain for every build.

Where another playthrough changes the picture

A replay is a good excuse to choose a different donor and see how the Kings’ cyberdog fits a different combat style.

His Search and Mark perk is simple but practical for exploration, especially when the player is sweeping cluttered interiors or open ground for useful items.

Rex begins as more than a novelty companion. The cyberdog is important to the Kings, and his declining brain creates the central problem in Nothin’ But a Hound Dog. The Courier is sent to find a viable donor, turning a companion recruitment story into a choice about what traits Rex will carry afterward. That decision gives the quest more weight than a simple “find replacement part” objective.

The three commonly pursued donor options point the player toward very different parts of the Mojave. Rey comes from Old Lady Gibson, Violetta is tied to the Fiends, and Lupa is found with Caesar’s Legion. Each replacement can alter Rex’s combat strengths, so the best choice depends on whether the player values durability, speed or damage rather than on one universally correct answer.

Search and Mark makes Rex useful even outside direct combat because nearby chems, weapons and ammunition containers become easier to spot. Combined with his speed and melee attacks, that gives him a different rhythm from ranged companions such as Boone. He is especially handy for players who like scavenging interiors and urban ruins instead of treating every companion primarily as another gun.

Rex also connects Freeside’s lighter personality to the wider faction war. The King’s affection for him humanizes a group that can otherwise be reduced to Elvis jokes and street politics, while one donor path reaches all the way into Legion territory. On a later playthrough, choosing a different brain is a compact way to see how a small companion quest can touch several communities without changing the main story route.

The donor decision is small compared with Hoover Dam, but it is exactly the kind of localized choice New Vegas handles well: a personal problem, three communities, and a mechanical consequence that follows the Courier afterward.

For a short companion quest, it packs an unusual amount of mechanical and faction context into one decision.