Theory: Donkey Kong is Cranky Kong's Grandson

Abstract
Cranky Kong is the Donkey Kong from the original ‘Donkey Kong’ arcade game. The modern Donkey Kong is Cranky’s grandson.

Arguments
"Whisking off maidens and throwing barrels around the place seven days a week, I was. That's how I got where I am today, you know. Hard work. None of this lazing around on the beach."
 * Argument 1: Cranky Kong has mentioned his glory days multiple times throughout the many games he has appeared in, with the biggest quote in favor of this theory being one of his lines from Donkey Kong Country 2: Diddy’s Kong Quest:

- Cranky Kong


 * Another moment implying Cranky’s past happens in the opening of Donkey Kong Country. Cranky Kong stands around in the girders of Donkey Kong playing the original Donkey Kong song before Modern Donkey Kong comes in to crash the party. If modern DK was the same Donkey Kong as the original DK, this moment would make no sense.
 * And the final major piece of evidence is in the instruction manual for Mario vs Donkey Kong 2: March of the Minis! Here, it states that DK instantly fell in love with Pauline when he saw her for the first time in this game, implying that he cannot be the original DK.


 * Argument 2: While it may seem strange that the original DK has aged so rapidly compared to Mario, this actually makes sense when you consider that, in-universe, the Kongs are shown to age rapidly, even for apes. This is seen the most with Tiny Kong, who went from looking tiny, to growing up into an older looking kong in the space of two games. This explains Cranky’s rapid aging.

"I'm pretty sure when I made this stuff up nearly 25 years ago that he was his grandson. By DK64 he was so senile he couldn't remember"
 * Argument 3: Throughout the Rare trilogy, Cranky was consistently referred to as DK’s grandfather. However, because Cranky Kong called the modern DK “son” in Donkey Kong 64, the idea that Cranky is DK’s father and therefore DK is actually Donkey Kong Jr. has become more prevalent. Once Rare left Nintendo, the confusion grew even more to the point that even Nintendo couldn’t seem to decide the truth. However, Gregg Mayles later came out and explained the Donkey Kong 64 discrepancy with this tweet:

- Gregg Mayles


 * And once Retro Studios took over the the country games, Cranky became DK’s grandfather once more, as we can see from both the Donkey Kong Country Returns Manual and the Donkey Kong Country: Tropical Freeze character trailer for Cranky Kong.