Kangaskhan did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Kangaskhan, known in Japan as ガルーラ (Garura), is a Normal-type, female-only species of Pokémon. While it has no known evolution or pre-evolution, it is known to Mega Evolve into Mega Kangaskhan, and to Mega Evolve, Kangaskhanite is the required Mega Stone to hold.
Kangaskhan is a large, bipedal creature with a thick tail and bumps on its back. It has a black, plate-like forehead and cream-colored auricles attached on each side of its head. Kangaskhan also has plate-like patches on its legs and shoulders. It has red pupils and white sclera. It has a cream-colored belly and pouch. This pouch usually contains its child, which it is very protective over.
The child Kangaskhan is smaller, has dot eyes, smaller teeth, a lavender skin and no patches or bumps over its skin, but looks for the rest roughly the same. Due to the parent Kangaskhan's protective nature, the child rarely gets out of the pouch unless it's over three years old. Sometimes, the parents lets the child out to play when it's safe, but still keeps a close eye on the child to make sure nothing happens to it.
When it Mega Evolves, Kangaskhan itself doesn't alter; instead, it's the child that gets the treatment. It grows up and is allowed to get out of the pouch and fight alongside its mother, which makes Mega Kangaskhan unique alongside other Mega Evolved Pokémon. While still having the lavender coloration, the child Kangaskhan gets the plate-like patches on its knees, just like its mother, as well as a ring of patches over its entire abdomen and hind. It grows spikes under its auricles and another one on its tail. Its eyes are now oval-shaped, as they go over the plate-like forehead. They eyes are no longer dot eyes, as they have white sclera, red irises and white pupils.
The Mega Evolved child is energetic, hates to lose, and challenges other opponents to protect its mother.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
It has been theorized that Cubone is actually the baby in Kangaskhan's pouch, due to their similar heads, the fact that Cubone lost its mother, and that Cubone shares its coloration with Kangaskhan. However, there are still a few contraries in this theory, such as Cubone's skull has horns, while Kangaskhan lacks horns and has ears instead.