Monkey Sage Mode is one of the few known Sage Modes in the ninja world. In the Warning States Period there was this jungle that was full of monkeys of all sizes. But one of the known monkeys was Son Goku, the four tails, he was the sage of the jungle and the king. Every monkey came from him and that how the monkeys were able to obtain Chakra, he would teach them how to use chakra, and eventually Senjutsu when they were ready. As time went on, the jungle stayed peaceful until, humans knew about Son Goku and came for him in the jungle. The humans would kill monkeys and cut down the jungle as they were on the search for Son Goku.
But, Son Goku would foil the Human's plan and kill them all the time. Then Son Goku had an encounter with a very powerful Human, and had to flee. At the jungle he gathered all the monkeys together, he then told them that the jungle would be called the Jungle of the Monkeys and gave some of his chakra to 9 monkeys, instead of the chakra being used up when the monkeys used the chakra that Son gave to them, the Chakra Son Goku gave to them would replenish. The monkeys he gave the power to would be known as Monkey Kings. Then he went to go and confront the powerful ninja and was caught. The 9 monkey kings then took the leadership and divided the whole jungle into 9 provinces, one for each Monkey King.
The 9 Monkey Kings then obtained peace until, the 5 great nation's were established. They knew that the humans would fight over the jungle, so what they did was kidnap a human weapon specialist and had him construct weapons from tools that were dropped by humans along the jungle. The Monkey Kings then used the weapons to make a technique that would summon the area of land to their choosing. They then found a canyon in the Land of Water, so they summoned the area of land their. From then on they lived a peaceful life. They would have some humans come and try to take them over or learn Sage Mode, but the Humans would fail and either leave or die.