Rex (Fallout: New Vegas)

"Second, he doesn't like hats, or the people wearing them."

Rex (or rather Mk. III Cyberhound, LEO Support Model, Serial no. B955883) is a 209 year old cyberdog living in Freeside in 2281 under the ownership of the King. He hates rats and will immediately go for one if he sees one. He will also bark at people wearing hats and it's unknown why, although the King says it might be because "hats" rhymes with "rats". He is suffering from neural degeneration as a result of his old age.

Background
Rex is a cybernetic dog that has been around since pre-war times. He was born as a regular dog in 2072 and served in the Denver City Police Department K-9 unit. Sometime before the war, he was cybernetically enhanced and turned into a cyberhound, although the circumstances of his cybernetic alterations remain unknown. He survived the destruction of Denver and survived the harsh wastelands, eventually becoming Caesar's companion. He later reached Freeside, where he came under the ownership of the King, who loves him dearly.

Capable of sniffing out loot and taking a large amount of punishment, he will join the Courier during the quest Nothin' But a Hound Dog. If the quest is completed, Rex can become a permanent companion. If Rex is fired at anytime, he will return to Freeside unless told to wait at the Lucky 38.

Four different dog brains can be used to complete the quest Nothin' But a Hound Dog. They are:
 * Lupa's brain from The Fort, which increases Rex's DT by 20 (1.2.0),
 * Rey's brain, which increases Rex's attack damage by 25 points (1.2.0),
 * An NCR guard dog brain, which grants no additional effect,
 * Violetta's brain (from the quest Three-Card Bounty), which increases Rex's speed by 50% (1.2.0)

Endings

 * If he never receives a replacement brain, Rex finally succumbs to old age, abruptly shutting down forever one quiet morning. After two centuries of life and decades of service to humanity, Rex collapses and dies.
 * If revitalized by Violetta's brain, Rex eventually learns to balance the memories of his old life with Violetta's experiences among the brutal Fiends. His mind has difficulty adjusting, but Rex eventually finds peace with his new, more vicious self.
 * If he receives a transplant of Lupa's brain, Rex gains all of the donor's experiences traveling with the Legion. These meld well with his own memories of the Legion, and his new mind quickly adjusts to the myriad memories.
 * If Rey's brain is transplanted into Rex's cybernetic body, it takes Rex some time to adjust to the old scrap yard dog's memories. Eventually, Rex's mind settles peacefully, melding his own memories with that of long travels with Old Lady Gibson.

Appearances
Rex appears only in Fallout: New Vegas.

Bugs

 * Sandboxing AI sees food (including water) on ED-E or Rex and decides that the only way to get that delicious food is to kill them. Because they are not NPCs (in code, they are "creatures"), this is "legitimate" behavior. Obsidian have fixed this behavior for the upcoming patch. Until then, a workaround is to remove food/water items from Rex/ED-E before hungry, hungry sandboxing folks get near those dudes.
 * If you dismiss Rex during Nothin' but a Hound Dog, when he is theoretically on loan to you and not a permanent companion, he may disappear and not return to The King. He may eventually show up after traveling to Doc Henry and speaking to him about Rex (dont't travel from The King to Henry and back, Rex still won't be there! When you realise he's gone, load the last saved game, go to Jacobstown and then visit The King).
 * When you tell Rex to wait at the Lucky 38, he can sometimes return to Freeside instead.
 * While you may be able to get Rex as a companion, you will not receive the quest Nothin' But a Hound Dog. Subsequently you will be unable to repair Rex's brain.
 * After replacing Rex's brain, he may become immobile.
 * When asked to wait, Rex may disappear after the player leaves the area. NPC's will still make remarks to you as if he was there. This can be fixed by returning to Freeside and waiting for 3 days. If this happens during Nothin but a Hound Dog you will have to wait to be able to finish Guess Who I Saw Today, if Rex is not returned by the time Lily needs to be tested and you have gotten a brain, Lily will not be tested until Rex receives his new brain.
 * After replacing Rex's brain with Lupa's, his health was set to 540/440 (440/440 being the default). Upon taking damage that reduced his health beneath 440, it could not be restored past 440. There is no known way to fix this bug. This issue remains unaddressed by the developer as of the 1.2.0285 update.
 * If Rex is with you when you talk to Corporal Sterling in Camp McCarran then he will attack the First Recon group. This is either a bug or due to his dislike of hats.
 * Rex may randomly attack people although they will not turn hostile towards you only rex. Notable examples are Quartermaster Bardon at Hoover Dam and the inhabitants of the Aerotech Office Park.
 * During combat, Rex might chaotically run around from-and-to foes while not attack.
 * After Rex joins you, he will sometimes refuse to attack regardless of what state or setting you choose for him. He will continue to remain passive throughout the game.  There is an easy fix for this, bring up the console with the ~ key, click on Rex, then type in "Kill". After Rex dies, bring up the console again, click on Rex, then type "Resurrect" and Rex will come back to life able to attack things again.
 * Rex will sometimes stop moving as if he has been told to wait. You will need to keep checking to see if he is still with you. This can usually be fixed by telling him to wait, then tell him follow you again, fast traveling or going through a door that requires a transition. However these fixes are per incident and he will randomly do it again. (Verified)
 * If you talk to Rex as soon as you get him for Nothin' But a Hound Dog, and the Perk for having him with you pops up, you will enter chat with him, but will have no options to talk. (Screen zooms in, he turns towards you, and no chat options appear.)
 * While doing the quest You Can Depend on Me (or just trying to sneak into the Gun Runners), Rex may glitch while in combat with the guards and end up in the Vendortron's kiosk. There is no physical way into the kiosk and Rex can still be spoken to. The only way to fix this problem is to fast travel to another loation, in which Rex will appear beside you in patched versions of the game.
 * While vilified by the Legion, if Rex simply kills Lupa while you are disguised and have her brain transplanted by Calamity, the epilogue may state that Rex never got a replacement brain even though the quest is completed, rather than the correct ending.
 * Wearing a set of Biker Goggles and talking to Rex will trigger dialog where he growls at you as if you were wearing a hat.