Zwei

Zwei is the pet dog of Ruby Rose and Yang Xiao Long. He first appeared in the episode "Field Trip", when he was mailed to Beacon Academy from Patch by Taiyang Xiao Long along with a pile of canned dog food and a can opener.

Taiyang sent Zwei to his daughters to take care of him temporarily due to him leaving Patch for undisclosed reasons.

After the Fall of Beacon, he went back to Patch with Yang and Taiyang. He is seen sleeping at home and later actively watching the two spar.

Appearance
Zwei is a black-headed tricolor Pembroke Welsh Corgi with gray eyes. Like some Pembroke Welsh Corgis, he has a bobbed tail, which is nowhere near the length that would indicate him as being a Cardigan Welsh Corgi. As of "Of Runaways and Stowaways", he wears a red collar with a pendant bearing two dots and a bone.

Personality
Zwei exhibits a very cheerful and playful personality, as seen when he licked Ruby and barked happily after being cuddled by Weiss Schnee. He also playfully imitates Yang's behavior.

Zwei also attempts to make friends with Blake Belladonna, even though her feline Faunus instincts cause her to react with some nervousness to his presence.

Zwei is shown to be very intelligent for a dog. Not only does he react appropriately to Ruby's commands, he also seems to be smart enough to use mechanical tools. When Weiss wonders how Zwei will eat his canned dog food and a can opener drops out of his tube, Yang cheerfully drops the issue, implying that Zwei can use it.

Zwei immediately formed a close relationship with Bartholomew Oobleck, refusing to leave his side during his fight against White Fang soldiers in "No Brakes".

Powers and Abilities
Zwei is a very intelligent dog, implied by Yang to be capable of using mechanical tools such as can openers, and responding correctly to Ruby's questions when needed. As a dog, he has the requisite heightened senses of smell and hearing, which allowed him to find the White Fang soldiers in "Mountain Glenn".

Likely due to his Aura, Zwei has been shown to perform feats no normal dog can do. He was first introduced compressing himself into a tube alongside dozens of cans of dog food and a can opener for an undisclosed amount of time without any ill effects on his body.

Later, in "No Brakes", he is shown to be fireproof and very durable, as seen when he destroys an Atlesian Paladin-290 by being lit on fire and launched by Oobleck. This is demonstrated once more in "Breach", where he survives the train explosion by taking shelter inside a Paladin, then proceeds to headbutt and defeat a nearby Beowolf.

Trivia

 * His name is likely a reference to the Cowboy Bebop character Ein, who is also a Pembroke Welsh Corgi, though of a different coloring. Eins is the German word for "one" while Zwei is German for "two".
 * In the credits of "Breach", Mrs. PennyApple and Penny Layne Matthews are credited as voice actors for Zwei.
 * Kerry Shawcross was very adamant on having Zwei's butthole on the model for RWBY Chibi, due to its comedic value, going as far as to ensure it was included on Zwei's chibi merchandise. "Zwei Painting" on of the shorts in "Love Triangle", the 10th episode of RWBY Chibi, also focuses on Zwei's posterior.
 * Cinder Fall refers to Zwei as a Corgi in "Pillow Fight", the 19th episode of RWBY Chibi.