Leaf Monkeys are animals portrayed in many episodes. In each episode they are Dang's enemy. In the episode Mr. Big Brother, They were giving out Leaf Monkey plushies.
You first see Dang's reaction to Leaf Monkeys in Mr. T.V., where he saw Adam in a monkey costume. He was previously covered in leaves when Derby's science experiment of a twister in a box went wrong and it got let out. It picked up Adam in the Monkey costume that he was wearing because the mascot of the show was a Monkey, and it threw him out of the school through the window and he landed in a tree. When he got covered in leaves and fell out of the tree, he was coming back to the lab inside the school and Dang saw him. He approached him in an angry manner, when he started punching Adam because he thought he was a Leaf Monkey.
Dang dislikes Leaf Monkeys because they are always stealing his fancy hat. Dang cares for his fancy hat a lot. You can see this in Mr. Alligator when Ivy's quiz told her she should be a plumber. Ivy had no idea what this is and she thought it was someone who picked plums. When Dang saw her jumping up near a tree he asked, "Did a Leaf Monkey steal your fancy hat? You can borrow my fancy hat, unless you are a Leaf Monkey in disguise". Ivy replied "I'm not a Leaf Monkey in disguise". Dang then was about to hand her the fancy hat when he said "unless you are a-". Ivy cuts him off and says "I'm not a Leaf Monkey in disguise, claiming to not be a Leaf Monkey in disguise." Later when Ivy found out that plumbers don't pick plums. she left and Dang's fancy hat got taken off of his head by a Leaf Monkey in the tree. He then started kneeling on the ground and screamed "Leaf Monkey!" As you can see, Dang's fancy hat is his most prized possesion.