Lefty

Lefty is a baby flying bison who bonded with Kai following the attack on the Northern Air Temple by the Red Lotus.

History
Lefty found an unconscious Kai hanging upside down from a tree after the boy had been shot out of the sky by P'Li and woke him up by licking his face. As the young airbender regained his bearings, Lefty lowered himself, enabling Kai to hop on his back. The duo made it back to the temple, where Kai intended to sneak back in, though opted to follow the airship leaving with all the temple's inhabitants instead. Lefty trailed the ship for miles to a cave-complex before returning to the temple, where Kai hoped to find some reinforcements.

They noticed a slab of earth being bent out of the mountain side and Lefty took them closer to investigate. Noticing it was the work of Bolin in an attempt to save himself, Mako, Asami, and Tenzin from lava, Kai offered them a lift on Lefty. The young bison struggled to maintain altitude with the five people on his back, though managed to make it back to the base of Laghima's Peak, where he made a crash-landing.

After revealing the airbenders and Korra's location, Kai mounted Lefty again and they guided the other members of Team Avatar to the impromptu prison cave. Upon reaching the cave, Lefty was left outside with Oogi and Tenzin while the others rescued the airbenders. Upon seeing Korra struggle to fight Zaheer, Kai offered to help Korra while riding Lefty, though Lin dismissed this idea since they would be no match for Zaheer's power.

Abilities
Despite still being a bison calf, Lefty is already capable of flight and has enough strength to carry five people on his back. He is also already capable of correctly responding to Kai's commands.

Trivia

 * Lefty responds to the command "yip yip", just like Oogi and Appa.