Kaa

Kaa the Rock python was Mowgli's wise mentor. He, Baloo and Bagheera sang for Mowgli "The Outsong" of the jungle. Kaa was a huge and powerful snake, unusually long, more than a hundred years old and still in his prime. Bagheera and Baloo enlisted Kaa's help to rescue Mowgli when the man-cub was captured by the Bandar-log and taken to an abandoned human city. Kaa broke down the wall of the building in which Mowgli was imprisoned and used his serpentine hypnosis to draw the monkeys toward his waiting jaws. Bagheera and Baloo were also hypnotized, but Mowgli was immune because he was human, and broke the spell on his friends.

Years later Kaa convinces Mowgli to visit a treasure chamber guarded by an old cobra beneath an ancient city. The cobra tried to kill Mowgli but its poison had dried up. Mowgli took an ankus from the treasure to keep as a weapon.

In later years Mowgli asked Kaa for help when his wolf pack is threatened by rampaging dholes (the red dogs of the title). Kaa goes into a trance so that he can search his century-long memory for a stratagem to defeat the dogs.

With Kaa's help Mowgli tricked the dholes into attacking prematurely. Kaa took no part in the resulting battle, but Mowgli and the wolves finally killed all the dhole, though not without grievous losses.

Trivia

Dick and Do/Tom and Tab the Hyenas from Kimba the White Lion resemble Kaa, except in Aryll's New World from Astro Boy Productions, where they're renamed Gal and Cal.
The Disney Kaa seems to be modeled after the reticulated python more than the Indian python, as the reticulated has in fact been known to eat humans (or try to).

Non-canon appearances

In the 1967 Disney movie, Kaa is a hungry giant snake who is nothing like the original version (this was most likely due to the developers' belief that the public of the time would not accept a snake as a heroic character). Instead of being a wise mentor to Mowgli he is portrayed as bumbling and cowardly villain, possibly meant to be a reticulated python which are known to treat humans as food (but Indian pythons never do). Kaa tries (and fails) to eat Mowgli on two different occassions, only to be foiled by Bagheera and Shere Khan. In the sequel, he tries to eat Shanti, however, he is foiled by Mowgli's former foster brother, Ranjan. Kaa was voiced by Sterling Holloway in The Jungle Book and is currently voiced by Jim Cummings. Cumming's portrayal receives criticism due to the voice being quite too similar to his Winnie-the-Pooh voice.
In the 1994 live-action Disney version, Kaa serves as a minion of King Louie who summons him by clapping 9 times. He guards the treasure of the monkey city from thieves and attacks and kills intruders when summoned. He's also far more aggresive and menacing in this version than any other.
In Disney's 2016 CGI remake and Warner Bros' Mowgli (Film), Kaa is once again portrayed as a villain. But this time he is depicted as female.
In the related television series Jungle Cubs, Kaa is portrayed as a baby snake and even friends with the other animal-characters.
Kaa also appears in Shonen Mowgli, and is more like the Kaa in the books. He is a mentor to Mowgli, but instead of resembling a python, he resembles a large green viper-like snake (though he is clearly a python by size and weaponry).
Kaa also appears in the Russian version Adventures of Mowgli where like in the anime, this Kaa is also a mentor to Mowgli. Voiced Vladimir Ushakov in Russian, Sam Elliott in English.
Kaa also appears in the 2010-CGI-TV-Series which also portrays him as an ally for Mowgli.