Make sure you have built enough speed and land in the quadrant depicted in the image.
Mystery of the Monkey Cage is the second mission of Tall, Tall Mountain.
Monkey Glitch Tutorial
After choosing to "Hold on" to the monkey on flat ground, you want to time landing on the slope with the next text prompt and choose "Free him"
Corner the monkey against a wall and he'll jump over you and pause, giving you time to grab him
To catch the monkey quickly, just chase him up to a wall and do the sideflip motion and punch him after you turn around.