Hellhound

' The Hellhounds' are a type of enemy first seen on Shi No Numa. They are seen on Shi No Numa, Der Riese, Kino der Toten, moon, The Giant, Der Eisendrache, and all Green run maps on Black ops 2 (if toggled on). They are zombie-dogs that are the mutated version of Samantha's pet, Fluffy and her offspring. They randomly spawn throughout the map (a ball of lightning will usually indicate whenever they spawn) every 4 to 7 rounds and their sole objective is to kill the player by streaking towards them and either attacking with tooth and claw or by exploding. The ground shakes whenever they spawn. There are two variants of the Hellhound: the flaming Hellhound, which on top of being perpetually on fire, explodes on death, and the normal Hellhound.

History
On Der Riese, there are some of radios that explain the Hellhounds' creation. Dr. Ludvig Maxis and his assistant, Edward Richtofen were doing tests at Der Riese. One of the tests involved Samantha's dog, Fluffy. When teleported, Fluffy became a Hellhound. When this occurred, Richtofen followed through with his plot to get rid of Maxis and his daughter, Samantha, by locking them in the room with the Hellhound, Fluffy. Dr. Maxis was killed while Samantha somehow survived, possibly due to the experiments done on her by Richtofen. Samantha seems to now control the Hellhounds as she attempts to kill Richtofen. One of the radio messages tells that Fluffy was pregnant, explaining why there are multiple Hellhounds.

The reason the Hellhounds are replaced by the Pentagon Thief on "Five" is because Samantha controls the Hellhounds and is only after Richtofen for what he did to her and her father. The Zombies attacking the Pentagon appear to have been led there by the Pentagon Thief, meaning the attack occurring there is unrelated to Samantha. Also of note is the fact that "Five" and Ascension occur at the same time, meaning Samantha is already preoccupied with what is happening in Ascension. The reason why the Space Monkeys replace the Hellhounds in Ascension is because of the lack of Teleporters and Element 115.

In World at War, Black Ops I, and Black Ops III, Hellhounds are automatically part of the game. But, in Black Ops II, you can toggle on and off Hellhounds.

Trivia

 * The Hellhounds have several different appearances just like the zombies. Some are large and intimidating while others look frail and badly burned.
 * The Hellhounds appear approximately every five rounds, although they are guaranteed to appear somewhere from rounds four to seven. Note that if teams complete a Hellhound round, it cannot happen again for three rounds, at which point it returns to the 50/50 probability.
 * The Hellhounds are most likely based on the mythical Hellhound.
 * The last Hellhound will always drop a Max Ammo when killed.
 * Hellhounds will attack certain players and don't switch between targets until the one they're after is downed.
 * In Der Riese, if a player shoots at them they will lock on to the player that shot at them regardless of the distance.
 * In Der Riese, there is a door with a warning sign and a Hellhound head nailed to it. Inside on the floor there is a lot of blood; this is possibly where Dr. Ludwig Maxis and his daughter, Samantha, met their supposed demise.
 * If the player does the glitch on Vendetta to get the Flammenwerfer 35, go on the second window and let the patrol go past, burn the dog. When its dead, it resembles a Hellhound.
 * When playing on Der Riese, if a teleporter is nearby (when camping in the balcony by Teleporter C, for example.) during a Hellhound round, many Hellhounds will spawn out of the teleporter.
 * Also, on Black Ops DS, there is no specific Hellhound round. Hellhounds can appear from round five and have increasing likelihood of appearing each round. These Hellhounds will knock down barricades with their teeth.
 * Only in Der Riese, they can appear in normal zombie rounds.
 * Hellhounds were originally going to appear in Five, since there are character response quotes for them in the game's files. This can be seen here.
 * There was originally going to be music playing during a dog round when Map Pack 3 was released, but Treyarch scrapped the idea.
 * The Temple is the only Call of Duty: Black Ops (Nintendo DS) map that does not feature Hellhounds.
 * A part of Beauty of Annihilation plays at the end of a Hellhound round.
 * During a Hellhound round, one can hear a strange young girls voice speaking in an unintelligable whispering tone. This is most likely Samantha Maxis.
 * The Hellhounds get weaker for everygame. In World at War, they are all strong. In Black Ops and Black Ops 2, they are sometimes flaming, but the Black Ops Hellhounds are still stronger, and finally, Black Ops 3 Hellhounds are always on fire and are quite weak compared to the original World at War Hellhounds.