Rattly the Rattlesnake is a large green and red ringed snake whose body is coiled, allowing him to jump high through the air like a spring; despite being an inhabitant of Crocodile Isle, Rattly is allied with the Kong Family and is described as "rebelling" against the reptilian Kremling Krew by aiding them.
In Donkey Kong Country 2: Diddy's Kong Quest, Rattly, first appearing in the level "Topsail Trouble", would allow Diddy Kong and Dixie Kong to ride him after they release him from his crate. With his superb jumping ability, Rattly would allow the Kongs to reach high areas normally unreachable by them. Additionally, Rattly can jump on enemies that Diddy and Dixie wouldn't normally jump on (Zingers and Spinies for some examples).
Donkey Kong Country 2: Diddy's Kong Quest also featured an entire level dedicated to Rattly known as "Rattle Battle", which was also the first level that had the Kongs changing into an Animal Buddy by using an Animal Barrel.
Having apparently survived the exploding and subsequent sinking of Crocodile Isle after the events of Donkey Kong Country 2: Diddy's Kong Quest, Rattly returned in Donkey Kong Land 2, once more to aid Diddy Kong and Dixie Kong.
Though Rattly does not actually appear in Donkey Kong Country 3: Dixie Kong's Double Trouble, his Animal Crate can be seen in the background of the level "Barrel Shield Bust-Up", which is located in the area Kremwood Forest.
List of appearances in Donkey Kong levels
Donkey Kong Country 2: Diddy's Kong Quest
Donkey Kong Land 2