Rattly the Rattlesnake 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):
Rattly the Rattlesnake is an Animal Friend that appears in the Donkey Kong series. He is a green, red-ringed rattlesnake and an ally to the Kongs.
Rattly's only appearance as an Animal Friend is in Donkey Kong Country 2: Diddy's Kong Quest. When broken out of an Animal Crate, he can be ridden on. Because he is always coiled in the shape of a spring, he has the ability to bounce extremely high, allowing him to find secrets or reach previously inaccessible areas. He can also store energy for an even higher bounce. Rattly also has the ability to bounce on enemies the Kongs previously couldn't damage, such as Zingers. Because of his abilities and immunity to hostile enemies, Rattly can be seen as an evolution of Winky the Frog from Donkey Kong Country.